i can't use my Voidal Resonator mount

i used the item to get him prior to unlocking the mount menu, so now i can't use it. any help will be apprciated

That sounds odd, even using the item before unlocking mounts it should still appear in your mount list once you finally unlocked the ability to use them when you join a Grand Company and get your company chocobo.

Have you tried opening the mount menu and then selecting the mount and dragging it into your hotbar and calling it that way? Also this may sound dumb but, are you trying to call it inside a city? You can't use mounts inside the city zones (although you can use them inside the residential areas).

That's weird. Try seeing if you can get it back from a calamity salvager.