"Do you buy milk at the store?"
Translation:Kupuješ mléko v obchodě?
I am afraid we need the entire sentence. We accept 2209 czech variations for this sentence, including many that start with 'kupujete'...
There are atm 2209 sentences that are accepted, including yours. File a bug report next time.
Unfortunately, we can only say that it is among the forms to be accepted. But people report it as not accepted from time to time. I will try to do some modification to reset the process, but who knows where the problem is. You can file a bug report with Duolingo.