Getting a custom plate doesn't mean you can just use whatever you want as the plate. All a vanity plate gets you is a combination of characters of your choosing - which then have to be approved. You still have to use one of the plate styles offered by the DMV.
You should use this site to check if the combination you want is available (apparently yours is) and then you're going to have to pick one of the plates they offer.
https://services.flhsmv.gov/MVcheckPersonalPlate/