Correct JSON format for a Tableview

Here is my code

import UIKit

struct MyList: Decodable {
    let ID: Int
    let columnaID: Int
    let columnb: String
    let columnc: String
    let columnd: String
    let columne: String
    let columnf: String
    let lat: Double
    let lon: Double
}

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        let myapiurl = "https://xxx-api.ap-southeast-2.amazonaws.com/xxxxxxxxxxxxxx"
        let urlObj = URL(string: myapiurl)
        
        URLSession.shared.dataTask(with: urlObj!) {(data, response, error) in
            
            do {
                var List = try JSONDecoder().decode([MyList].self, from: data!)
                
                for columnb in List {
                    print(columnb)
                }
                
            } catch {
                print("We got an error")
            }
            
            }.resume()
    }
    
}

That is giving me error at line 27 Variable 'List' was never mutated; consider changing to 'let' constant

I have been given an api which outputs the following

You should better add how you have output that JSON, which prevents some sort of easy misunderstanding.

I assume it is the whole response from your API.

And with pretty-printed:

"""
{
    "statusCode": 200,
    "headers": {
        "Access-Control-Allow-Origin": "*"
    },
    "body": [
        {
            "ID": 29,
            "ColumnAID": 12,
            "ColumnB": "Blah",
            "ColumnC": "",
            "ColumnD": "Not Listed",
            "ColumnE": null,
            "ColumnF": null,
            "lat": null,
            "lon": null
        },
        {
            "ID": 22,
            "ColumnAID": 12,
            "ColumnB": "Blah",
            "ColumnC": "",
            "ColumnD": "Canberra",
            "ColumnE": "ACT",
            "ColumnF": null,
            "lat": null,
            "lon": null
        },
        {
            "ID": 24,
            "ColumnAID": 12,
            "ColumnB": "Blah",
            "ColumnC": "",
            "ColumnD": "Bondi Junction",
            "ColumnE": "NSW",
            "ColumnF": null,
            "lat": null,
            "lon": null
        },
        {
            "ID": 25,
            "ColumnAID": 12,
            "ColumnB": "Blah",
            "ColumnC": "",
            "ColumnD": "Broadway",
            "ColumnE": "NSW",
            "ColumnF": null,
            "lat": null,
            "lon": null
        },
        {
            "ID": 26,
            "ColumnAID": 12,
            "ColumnB": "Blah",
            "ColumnC": "",
            "ColumnD": "Castle Hill",
            "ColumnE": "NSW",
            "ColumnF": null,
            "lat": null,
            "lon": null
        }
    ]
}
"""

Outlined:

{
    "statusCode": 200,
    "headers": {...}
    "body": [...]
}

The outer most structure `{...}`is a JSON object, not an array. You may need to prepare something like this:

struct MyResponse: Decodable {
    let statusCode: Int
    let headers: [String: String]
    let body: [MyList]
}

struct MyList: Decodable {
    let ID: Int
    let columnaID: Int
    let columnb: String
    let columnc: String
    let columnd: String
    let columne: String?
    let columnf: String?
    let lat: Double?
    let lon: Double?
    
    enum CodingKeys: String, CodingKey {
        case ID
        case columnaID = "ColumnAID"
        case columnb = "ColumnB"
        case columnc = "ColumnC"
        case columnd = "ColumnD"
        case columne = "ColumnE"
        case columnf = "ColumnF"
        case lat
        case lon
    }
}

And try this:

            do {
                let myResponse = try JSONDecoder().decode(MyResponse.self, from: data!)
                let myList = myResponse.body
                
                for listItem in myList {
                    print(listItem.columnb)
                }
                
            } catch {
                print("We got an error")
            }

(You should better add error checking, but that's another issue.)