In some of my personal symbols, when running the symbol checker it appears “Reference prefix ending by ‘0123456789?’ can create issues if saved in a symbol library”. I don’t know where this prefix comes from and how to overcome the warning.
It’s a symbol whose reference prefix ends with a digit or question mark. That causes problems with annotations.
Look through your symbol library. One that you made or imported?
This usually occurs if you load a symbol from the schematic into the Symbol Editor. In that case the reference sesignator is often allready filled in / annotated. If you then save it in a (personal) library, you should first remove the annotated number from the RefDes.