How to fix 'Cannot use mutating member on immutable value: 'self' is immutable' error in Xcode

I create a random number and assign it to the variable randomNumber. Then when I try to append the random number as an index for the emojis array to the empty array arrayOfEmojis I get the error:

'Cannot use mutating member on immutable value: 'self' is immutable'

Im also getting errors from the emoji?

import GameKit

struct EmojiProvider {
    var emojis = ["😀", "😃", "😄", "😁", "😆", "😅", "😂", "🤣", "☺️", "😊", "😇", "🙂", "🙃", "😉", "😌", "😍"]
    var arrayOfEmojis: [String]

    func generateEmoji() -> [String] {
        for i in 1...3 {
            var randomNumber = GKRandomSource.sharedRandom().nextInt(upperBound: emojis.count)
            arrayOfEmojis.append(emojis[randomNumber]) //error!
        }
        return arrayOfEmojis
    }
}

This happens because when you have a struct, instead of a class, you're mutating the instance itself.
Hence, when you're trying to mutate self, mark the function as mutating:

mutating func generateEmoji() -> [String] {

Also, I suggest you select a random element as follows:

return (1...3).compactMap { _ in emojis.randomElement() }

...and leave our the whole arrayOfEmojis, if you don't want to access it later.

The full code would then be:

struct EmojiProvider {
    var emojis = ["😀", "😃", "😄", "😁", "😆", "😅", "😂", "🤣", "☺️", "😊", "😇", "🙂", "🙃", "😉", "😌", "😍"]

    mutating func generateEmoji() -> [String] {
        return (1...3).compactMap { _ in emojis.randomElement() }
    }
}

What you could also do, is make EmojiProvider a class, instead of a struct.
You could then have the following code:

class EmojiProvider {
    var emojis = ["😀", "😃", "😄", "😁", "😆", "😅", "😂", "🤣", "☺️", "😊", "😇", "🙂", "🙃", "😉", "😌", "😍"]

    func generateEmoji() -> [String] {
        return (1...3).compactMap { _ in emojis.randomElement() }
    }
}