Have you tried using /prefixlist? That command shows you all your available prefixes and you may just have Ace turned off. If there is a bigger issue you might wanna clarify which server this is on so staff can help you out.
Apologies for the inconvenience! As squidsi mentioned, if you purchased a rank in the past, it should appear in the command /prefixlist in the respective server that you purchased the rank one. If you are unable to find the rank on the server that you purchased it on, please feel free to let us know your IGN and the server on this forum thread or by creating a ticket in the #help-and-support channel on our Discord and one of our staff members will help you out as soon as possible! If you have any other questions, please feel free to let us know and we will help you out right away. Stay safe and healthy.