This simply means that you did not gamble at all on this casino?
If the above question be true, then I assume you deposited the $500 for the sole purpose of claiming a deposit bonus, which some casinos can consider as an abuse of their bonus offer, and block the user..
Several casino have a wagering requirement, which is a percentage that is required that the user wager (after depositing) before such user will be able to withdraw, so if like you said, you simply deposited, claimed a deposit bonus, then straight away requested a withdrawal, it simply means you were abusing the deposit bonus and I believe that was why you were blocked.
I will you keep chatting with their customer support, hopefully, they will answer you, you can also pm the casino representative here, though inactive for some time now, but hope he or she comes online and see how they can help you.