"What are you hiding?"
Okay, new problem. When using the 'keyboard and definition' setup to translate "what are you hiding" into Latin, "what" becomes "quid". Okay... "hiding (singular, you are doing it)" becomes "celas". Again, okay... Now, "are you" is translated as "es tu". Makes sense, right? Well, not to Duo, who will mark you as WRONG and explain that the REAL answer was a simple "quid celas" instead of "quid es tu celas". Question: why is "quid es tu celas" wrong?
Because celas can be translated itself to 'you are hiding', 'you hide', 'you do hide'. The present active indicative (and many/most tenses) are not compound tenses and so not use a form of esse. Adding es here would give more incorrect English as 'you are are hiding' or 'you are hide'.
Edit: adding tu -> quid tu celas? should work (just emphasis is put on the subject 'you').