Code Golf: Forwards sdrawkcaB sdrawkcaB Forwards Forwards sdrawkcaB
TeaScript, 55 bytes 58 60 69 76 78 80 87 89

xO`a-z `?xl(#~-i&amp;2?l:lv(),/ +/):Ld`SÀZn­ Û § «e Ò5s`


This is extremely short, I'm very happy with it.

The last ~20 characters may seem like gibberish but that's "Sentence must only use letters" encoded. All the characters have char codes below 256 so each are one byte

Explanation

xO`a-z `?  // If input contains only a-z and space...

   xl(#       // Loop through input 
      ~-i&amp;2?  // If (index - 1 "unary and"ed with 2) isn't 0...
          :l,     // Leave alone
          lv()    // Otherwise, reverse string
       / +/ // Loops on spaces
   )

:Ld`SÀZn­ Û § «e Ò5s` // Otherwise... decompress and print  the error string



Try it online
Pastebin