To be clear: does this mean you were able to resolve the issue for yourself?
If not, this error is being produced by outdated server managers in the library/Server/Manager
folder.
The easiest way to fix it would be to compare what's there VS what we officially include in FOSSBilling
FOSSBilling/FOSSBillingtree/main/src/library/Server/Manager
If something isn't listed in our source code and you aren't using it, make a backup of the file somewhere (outside of that folder) and then delete it