It’s not possible to move a candidate to a project where the candidate is already registered. Check the candidate profile to see if candidate already is already registered on the project.
Candidate may also be registered in the project even though you're not able to see the candidate being registered there. If candidate have started to fill out the application and saved, but not sent the application then candidate is logged in the project, however the application is inactive and not shown to users in system. If candidate have already started to fill the application then candidate must also make sure to complete and send in the application.