Eliminate array .sorted errors Swift 5

This is the expected behaviour. You are comparing strings, not numbers, and "Chapter 40" > "Chapter 5" == true as "4" > "5" alphabetically and they are the characters in the same positions in the strings.

If you want to retain the data as strings and order the way you want you will need a custom sorting function. Based on the example strings you could use something like

func bookLessThan(_ lhs: String, _ rhs: String) -> Bool {
   let lhsComponents = lhs.components(separatedBy: " ")
   let rhsComponents = rhs.components(separatedBy: " ")
   return lhsComponents.first! != rhsComponents.first! ? lhsComponents.first! < rhsComponents.first! :
   Int(lhsComponents.last!)! < Int(rhsComponents.last!)!
}

var chaptersAndUnits: [String] = ["Chapter 2", "Chapter 3", "Chapter 5", "Unit 12", "Chapter 40"]

let sorted = chaptersAndUnits.sorted(by: bookLessThan)

print(sorted)
// ["Chapter 2", "Chapter 3", "Chapter 5", "Chapter 40", "Unit 12"]

NB: In practice you'd want to safely unwrap all the optionals in case your strings aren't all in the expected format, and then handle that, maybe by returning a false value so they are all at the end.