Navigating between different views depending upon the button clicked

You should use isActive in the NavigationLink:

So, I would define the Link for each button (with an isActive),

NavigationLink(
    "Name x",
    destination: DestinationView(), // different for each
    isActive: $shouldFire[x])

have a State var (shouldFire, array of Bool or use each of State var as setMemory) to keep what is the button to fire, then turn on in Continue Button the right link.

Well, I had to restructure the code a little bit, making more use of State var, putting things in arrays, using ViewBuilder to select the appropriate destination…

Here it is (in 2 parts because of post limit):

struct MemoryView: View {
    var body: some View {
        Text("Memory")
    }
}

struct SocialView: View {
    var body: some View {
        Text("Social")
    }
}

struct ApproachesView: View {
    var body: some View {
        Text("Approaches")
    }
}

struct PsychopathologyView: View {
    var body: some View {
        Text("Psychopathology")
    }
}

struct BiopsychologyView: View {
    var body: some View {
        Text("Biopsychology")
    }
}

struct AttachmentView: View {
    var body: some View {
        Text("Attachment")
    }
}

struct IssuesandDebatesView: View {
    var body: some View {
        Text("IssuesandDebates")
    }
}

struct SchizophreniaView: View {
    var body: some View {
        Text("Schizophrenia")
    }
}

struct ResearchMethodsView: View {
    var body: some View {
        Text("ResearchMethods")
    }
}

struct ContentView: View {

    let kSetMemory          = 0
    let kSetSocialInfluence = 1
    let kSetApproaches      = 2
    let kSetPsychopathology = 3
    let kSetBiopsychology   = 4
    let kSetAttachment      = 5
    let kSetIssuesandDebates = 6
    let kSetSchizophrenia   = 7
    let kSetResearchMethods = 8

    @State var topicSelected : [Bool] = [false, false, false, false, false, false, false, false, false]

    @State var isActive = false // We can activate link
    @State var fire = false     // we fire link

    func resetOthers(except: Int) {
        isActive = topicSelected[except]    // The link will be activated
        if !topicSelected[except] { return }
        for k in kSetMemory...kSetResearchMethods where k != except {
            topicSelected[k] = false
        }
    }
    
    @ViewBuilder func destination(topic: Int) -> some View {
        switch topic {
            case kSetMemory:
                MemoryView()
            case kSetSocialInfluence:
                SocialView()
            case kSetApproaches:
                ApproachesView()
            case kSetPsychopathology:
                PsychopathologyView()
            case kSetBiopsychology:
                BiopsychologyView()
            case kSetAttachment:
                AttachmentView()
            case kSetIssuesandDebates:
                IssuesandDebatesView()
            case kSetSchizophrenia:
                SchizophreniaView()
            case kSetResearchMethods:
                ResearchMethodsView()
            default:
                EmptyView()
        }
    }

    private var topicNumber: Int? {
        for k in 0..<topicSelected.count {
            if topicSelected[k] { return k }
        }
        return nil
    }
    
    private var isFormValid: Bool { // One and only one selected
        topicSelected.filter({ $0 }).count == 1
    }

And the second part:

    var body: some View {

        NavigationView {

            ScrollView {
                    //Used Group{} to Prevent Argument Error

                Group {
                    VStack(alignment: .leading, spacing: 5) {
                        Toggle("Memory",isOn: $topicSelected[kSetMemory])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.902, green: 0.755, blue: 0.161))
                            .onChange(of: topicSelected[kSetMemory], perform: { value in resetOthers(except: kSetMemory) } )
                        
                        Toggle("Approaches",isOn: $topicSelected[kSetApproaches])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.945, green: 0.442, blue: 0.022))
                            .onChange(of: topicSelected[kSetApproaches], perform: { value in resetOthers(except: kSetApproaches) } )
                        
                        Toggle("Biopsychology",isOn: $topicSelected[kSetBiopsychology])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.817, green: 0.065, blue: 0.287))
                            .onChange(of: topicSelected[kSetBiopsychology], perform: { value in resetOthers(except: kSetBiopsychology) } )
                        
                        Toggle("Issues & Debates",isOn: $topicSelected[kSetIssuesandDebates])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.399, green: 0.06, blue: 0.947))
                            .onChange(of: topicSelected[kSetIssuesandDebates], perform: { value in resetOthers(except: kSetIssuesandDebates) } )
                        
                        Toggle("Research Methods Year 1 & 2",isOn: $topicSelected[kSetResearchMethods])
                            .toggleStyle(.button)
                        .tint(Color(red: 0.105, green: 0.561, blue: 0.896))
                        .onChange(of: topicSelected[kSetResearchMethods], perform: { value in resetOthers(except: kSetResearchMethods) } )

                    }
                    
                    .padding(.leading, -135.0)
                    .padding(.top, -10)
                    
                    VStack(alignment: .leading, spacing: 5) {
                        
                        Toggle("Social Influence",isOn: $topicSelected[kSetSocialInfluence])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.902, green: 0.755, blue: 0.17))
                            .onChange(of: topicSelected[kSetSocialInfluence], perform: { value in resetOthers(except: kSetSocialInfluence) } )

                        Toggle("Psychopathology",isOn: $topicSelected[kSetPsychopathology])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.945, green: 0.442, blue: 0.022))
                            .onChange(of: topicSelected[kSetPsychopathology], perform: { value in resetOthers(except: kSetPsychopathology) } )

                        Toggle("Attachment",isOn: $topicSelected[kSetAttachment])
                            .toggleStyle(.button)
                            .tint(Color(red: 0.817, green: 0.065, blue: 0.287))
                            .onChange(of: topicSelected[kSetAttachment], perform: { value in resetOthers(except: kSetAttachment) } )

                        Toggle("Schizophrenia",isOn: $topicSelected[kSetSchizophrenia])
                            .toggleStyle(.button)
                        .tint(Color(red: 0.394, green: 0.061, blue: 0.943))}
                    .onChange(of: topicSelected[kSetSchizophrenia], perform: { value in resetOthers(except: kSetSchizophrenia) } )

                    .padding(.top, -192)
                    .padding(.leading, 180)
                }
                
                Spacer()
                Spacer()

                Button(action: {
                    fire = true
                }) {
                    NavigationLink(destination: destination(topic: topicNumber ?? 10), isActive:  $fire) { // 10 just to have value
                        Text("Continue")
                    }
                    .padding(.vertical, 12.5)
                    .padding(.horizontal, 120)
                    .foregroundColor(.white)
                    .foregroundStyle(.background)
                    .background(isFormValid ? Color.red: Color.gray)

                    //'Continue' Button is Disabled if User Has Not Selected Values
                    .clipShape(Capsule())
                }
                .disabled(!isFormValid)
            }

            .navigationTitle("Select domain")
            .padding(.top, 20)
        }
    }

Thankyou so much for this! I really appreciate it :)