Like the fixed fin came unglued?

I didn't find a replacement fixed fin in a quick amazon search, but you can certainly glue on a standard fin box and use a regular fin with it. If it's the center fin that you lost it's a quick prep/clean/glue job to replace it with something like this https://www.amazon.com/Jashem-Inflatable-Paddleboard-Replacement-Surfboard/dp/B0BX4MPPCW

If it's a missing side fin, then you will need to do a bit more to get it fixed well. If that's the case I would remove the center fin, glue it where the side fin was, and then replace the center fin as above.