Everytime I use sleep talk on snorlax he uses rest and fails. Is this a glitch?

Sleep talk will select one of your moves that isn't sleep talk and use that. If it selects Rest while you are already asleep it will fail. That's how it's supposed to work. In that way, using Sleep Talk will do nothing 33% of the time because Rest is selected.

 

If you had other moves besides Sleep Talk and Rest that weren't ever selected it would seem bugged, since there should be an equal chance of 33% for one of the 2 remaining moves to be selected. Otherwise no.