function with automatically increasing percentage (watchOS)

I think there's a bit too much code here for us to figure out what's going on. Can you provide a minimum reproducible example? How about explaining exactly where things are going wrong?

Also, "Already tried to get help from AI, but did not solve the problem." Don't resort to "AI" to fix your problems. It doesn't work. Large Language Models work on patterns, and aren't really going to figure out where you're going wrong with this code.

If I had to guess, I would say somerewehre right there's the mistake or something's missing:

    ]
    
    let maxECTSPerSection: [CourseCategory: Int] = [
            .firstSection: 44,
            .secondSection: 172,
            .freeStudy: 24
        ]

        init() {
            loadCourses()
        }

        var progress: Double {
            let completedECTS = courses.filter { $0.isCompleted }.reduce(0) { $0 + $1.ects }
            let totalECTS = 240 // Gesamtwert für ECTS im Studium
            return (Double(completedECTS) / Double(totalECTS)) * 100
        }

    func toggleCourseCompleted(courseID: String) {
        if let index = courses.firstIndex(where: { $0.id == courseID }) {
            courses[index].isCompleted.toggle()
            saveCourses()
        }
    }

    private func completedECTS(for category: CourseCategory) -> Int {
        let completedECTSAsDouble = courses.filter { $0.category == category && $0.isCompleted }.reduce(0.0) { $0 + $1.ects }
        return Int(completedECTSAsDouble)
    }
    }

    // Extension außerhalb der Klassendefinition
    extension StudyProgressViewModel {
        func saveCourses() {
            do {
                let encodedCourses = try JSONEncoder().encode(courses)
                UserDefaults.standard.set(encodedCourses, forKey: "savedCourses")
            } catch {
                print("Fehler beim Speichern der Kurse: \(error)")
            }
        }
        
        
        func loadCourses() {
            guard let savedCoursesData = UserDefaults.standard.data(forKey: "savedCourses") else { return }
            do {
                let decodedCourses = try JSONDecoder().decode([Course].self, from: savedCoursesData)
                DispatchQueue.main.async {
                    self.courses = decodedCourses
                }
            } catch {
                print("Fehler beim Laden der Kurse: \(error)")
            }
        }
    }
extension StudyProgressViewModel {
    // Ihre neue Funktion...
    func updateFreeStudyECTS(selectedCourses: Set<String>) {
        let selectedCoursesECTS = courses.filter { selectedCourses.contains($0.name) && $0.category == .freeStudy }
                                        .reduce(0.0) { $0 + $1.ects }
        print("Aktualisiere Free Study ECTS für ausgewählte Kurse: \(selectedCourses)")

        
        if selectedCoursesECTS <= Double(maxECTSPerSection[.freeStudy] ?? 24) {
            for course in courses where selectedCourses.contains(course.name) && course.category == .freeStudy {
                if !course.isCompleted {
                    toggleCourseCompleted(courseID: course.id)
                }
            }
        } else {
            print("Die Gesamtanzahl der ECTS für freie Studienleistungen darf 24 nicht überschreiten.")
        }
    }
}
extension StudyProgressViewModel {
    func isCourseCompleted(courseName: String) -> Bool {
        return courses.first { $0.name == courseName }?.isCompleted ?? false
    }

    func toggleCourseCompletedByName(courseName: String) {
            print("Looking for course: \(courseName)")
            print("Available courses: \(courses.map { $0.name })")
            if let index = courses.firstIndex(where: { $0.name == courseName }) {
                print("Course found: \(courseName), toggling completion")
                toggleCourseCompleted(courseID: courses[index].id)
            } else {
                print("Course not found: \(courseName)")
            }
        }
    }

Maybe somewhere in the extension, but not sure..

@pabra99 that's not a reproductible example. That's essential, to analyse what occurs.

If I had to guess, I would say somewhere right there's the mistake or something's missing:

Debugging is not about guessing, but about getting evidence.

maybe my friend, but since this is my first ever project, it's not as easy for me as it is for you to get some evidence what the problem could be :)