Compare Button Label value to string variable

I'm trying to write a simple quiz app. Upon a user tapping a button, I want the action to compare the Button's label to see if it matches the variable of "correctChoice"

I tried the following if/then statement but it doesn't seem to work, as I'm getting the following error "Left side of mutating operator isn't mutable: 'self' is immutable"

//Variables Used

var model = Question.all()
var questionNumber: Int = 1
var totalQuestions: Int = 100
var timeLeft: Int = 60
var currentScore: Int = 0

//Button Code

Button(action: {
                 if Question.all()[0].choice1 == Question.all()[0].correctChoice {
                            currentScore += 10
                        } else {
                            currentScore += 0
                        }  
                    }) {
                        Text(Question.all()[0].choice1)
                            .font(.system(size: 20))
                    }