Getting “mutating method sent to immutable object” error

Question

I can\'t figure out what is causing this. Basically, a few different \'tasks\' are colliding with each other in my app. When i press a button, it runs this code just fine:

Answer 1

TO REMOVE AN OBJECT FROM PARTICULAR INDEX OF AN ARRAY. (Swift 3.0)

let fullArray : NSArray = Userdefaults().value(forKey: "YOUR_ARRAY_STRING") as! NSArray
var mutableArray : [AnyObject] = fullArray as [AnyObject]
mutableArray.remove(at: INDEX_TO_REMOVE) //Eg: mutableArray.remove(at: 0)
mutableArray.append(ARRAY_TO_APPEND)