s.substring(0, s.length - x) replaceable with s.dropLast(x).
Using a corresponding functions makes your code simpler.
The quick-fix replaces substring call with dropLast call.
Example:
fun foo(s: String) {
s.substring(0, s.length - 5)
}
After the quick-fix is applied:
fun foo(s: String) {
s.dropLast(5)
}