Tested this from the reference: https://developer.apple.com/documentation/swift
var string = String(count: 5, repeatedValue: \"a\")
// string is \"aaaaa\"
It seems that you have to explicitly pass in a Character type to it to function. This works for me.
let char = Character("a")
let string = String(count: 5, repeatedValue: char)
Although, there may be bug mixed in with all this as well. I believe the way you were doing this should have worked on its own. And I can't seem to get code completion on this initializer at all.
Edit: I'm going with bug. The following compiles just fine.
let array = Array(count: 5, repeatedValue: "a")