Function to translate from English to Morse

I'm somewhat puzzled that OP hasn't written the, to my mind, rather simpler:

ToMorseCode[text_String] := StringReplace[
  ToLowerCase[text], {"a" -> ".- ", "b" -> "-... ", "c" -> "-.-. ", 
   "d" -> "-.. ", "e" -> ". ", "f" -> "..-. ", "g" -> "--. ", 
   "h" -> ".... ", "i" -> ".. ", "j" -> ".--- ", "k" -> "-.- ", 
   "l" -> ".-.. ", "m" -> "-- ", "n" -> "-. ", "o" -> "--- ", 
   "p" -> ".--. ", "q" -> "--.- ", "r" -> ".-. ", "s" -> "... ", 
   "t" -> "- ", "u" -> "..- ", "v" -> "...- ", "w" -> ".-- ", 
   "x" -> "-..- ", "y" -> "-.-- ", "z" -> "--.. ", " " -> "/ "}]

Use ToLowerCase[string] before applying your function.