This is mentioned several times in the tips and notes for various lessons -- "Sleep!" as a single-word sentence sounds like a command, but cysgu etc. are not command forms.
So to make sure that we understand that, the verb-noun form cysgu is expected to be translated as either "to sleep" or "sleeping", and similarly for all other verb-nouns.
Just plain sleep is not accepted for a single-word Welsh verb-noun prompt.
In a whole sentence, of course, just sleep may be correct, e.g. Dw i ddim yn gallu cysgu "I can't sleep".