What's Wrong Here: Instance Member Cannot Be Used on Type

Instance member cannot be used on type

You just have syntax error when saying = {return self.someValue}. The = isn't needed.

Use :

var numPages: Int {
    get{
        return categoriesPerPage.count
    }

}

if you want get only you can write

var numPages: Int {
     return categoriesPerPage.count
}

with the first way you can also add observers as set willSet & didSet

var numPages: Int {
    get{
        return categoriesPerPage.count
    }
    set(v){
       self.categoriesPerPage = v
    }
}

allowing to use = operator as a setter

myObject.numPages = 5

Instance member cannot be used on type?

it should be;

public mutating func setPrivateLogger(imp: ((_: String) -> Void)?) {
    logger = imp
}

explanation:
struct is a value type. Therefore, when you change it, you actually get a new value just like Int, String or another value type.
Because it is a value type, you cannot change it in its own instance.
"By default, the properties of a value type cannot be modified from within its instance methods" (Swift documentation)
To be able to do this, you need "mutating" behavior.

for more info:
https://docs.swift.org/swift-book/LanguageGuide/Methods.html

if you want to set "logger" only in Struct, you can define it as "private(set) var logger" or if you want to access it only in Struct, you can define it as "private var logger"

struct Console {
static let formatter = DateFormatter()
private(set) var logger: ((_: String) -> Void)?

public static func log(_ level: Slog_ClientLog.LogLevel,_ tag: String,_ message: String) {
    // other code
 }

public mutating func setPrivateLogger(imp: ((_: String) -> Void)?) {
    logger = imp
 }
}

What's wrong here: Instance member cannot be used on type

The problem here is that you are using self before the class is fully initialised. You can either have a getter which will be called every time you access the variable or compute it lazily.

Here is some code:

class TableViewController: UITableViewController {
    let mydate = NSDate()
    var items : [(Int,Int,Int,String,NSDate)] {
        get {
            return [
                (1, 9, 7, "A", mydate),
                (2, 9, 7, "B", mydate),
                (3, 9, 7, "C", mydate),
                (4, 9, 7, "D", mydate)
            ]

        }
    }
}

Lazy computation:

class TableViewController: UITableViewController {
    let mydate = NSDate()
    lazy var items : [(Int,Int,Int,String,NSDate)] =  {

            return [
                (1, 9, 7, "A", self.mydate),
                (2, 9, 7, "B", self.mydate),
                (3, 9, 7, "C", self.mydate),
                (4, 9, 7, "D", self.mydate)
            ]

    }()
}

How to fix Instance member cannot be used on type segmentedControl

You seem to have made a custom type named segmentedControl and calling the method on it. You need to remove or rename if there is such a declaration in your code. Better to rename it to MyCustomSegmentedControl from segmentedControl.

class segmentedControl: UISegmentedControl { ... } // rename or remove

You need to call the method on it's instance instead, like this:

let segmentedControl = UISegmentedControl()
segmentedControl.changeUnderlinePosition()

Instance member cannot be used on type - error

If you need to use the method without an instance, you need to declare the method as static.

static func checkInputs(_ user: Sign, opponent: Sign) -> String

Note: You do not seem to using the user that you pass. IMO you could skip asking for that parameter and use it as an instance method with playerChoice.

func checkInputs(opponent: Sign) -> String {
    // Your logic
}

And then use it like this

playerChoice.checkInputs(opponent: randomSign())

The second error is because you are trying to return an instance of Sign instead of a String. You need to either change the return type to Sign or covert the Sign in outcome to String - outcome.text like @Larme pointed out?

Instance member '....' cannot be used on type 'CustomTextField'; did you mean to use a value of this type instead?

I distilled the problem down a bit and recreated it, so you'll want to add back the code I removed. This code produces the same error:

class CustomTextField {
    var leftIcon: String = ""
    
    private let leftIconView: UIImageView = {
        let icon = UIImage(systemName: leftIcon) // <--- Error here
        let imageView = UIImageView(image: icon)
        return imageView
    }()
}

And tracked down the problem.

The problem is that initializing leftIconView is actually happening in a static context before the initializer is called, so you can't refer to instance properties. I found two easy solutions, the first is make leftIconView a lazy var instead of a let:

class CustomTextField {
    var leftIcon: String = ""
    
    private lazy var leftIconView: UIImageView = {
        let icon = UIImage(systemName: leftIcon)
        let imageView = UIImageView(image: icon)
        return imageView
    }()
}

The second solution is to just initialize leftIconView in the initializer.

class CustomTextField {
    var leftIcon: String = ""
    
    private let leftIconView: UIImageView
    
    override init()
    {
        let icon = UIImage(systemName: leftIcon)
        self.leftIconView = UIImageView(image: icon)
    }
}

This second solution will probably require that you also implement some other initializers for UIView once you add back that inheritance, and of course you'll need to call super.init at the end of the initializer.

Addendum

The above solution did indeed fix the compile error, but actually making the icon appear in the CustomTextField required using a property observer on leftIcon

class CustomTextField: UITextField, UITextFieldDelegate {
    //"person"
    private var showPassword: Bool = false
    var leftIcon: String = ""
    {
        didSet
        {
            leftIconView = makeImageView(for: leftIcon)

            // leftView is the view that is actually displayed.  It's set
            // once in the initializer, so now that we're updating 
            // leftIconView, we have to update leftView as well
            leftView = leftIconView
            
            // Maybe also do this
            leftView?.setNeedsDisplay()
        }
    }
    
    private lazy var leftIconView: UIImageView = {
        makeImageView(for: leftIcon)
    }()
    
    private func makeImageView(for imageName: String) -> UIImageView {
        let icon = UIImage(systemName: imageName )
        let imageView = UIImageView(image: icon)
        imageView.contentMode = .scaleAspectFit
        imageView.tintColor = .white
        return imageView
    }
    
    ...
}