I have this issue too. This project was great but obviously it's being abandoned, so sad :(
Hi Stylophone, I could fix it, I drop the Unity Folder placed on c:\User\Your PC USer\AppData\Local\ . Reinstall the package. Bye
Hi, thanks for let me know. The method you mentioned didn't work for me. The way I made it works is to delete both GameFlow.dll and GameFlowEditor.dll that imported in project, and renaming/reimporting them from the Versions/2019.3 folder (the GameFlow.bak and GameFlowEditor.bak). Just for share in case anyone needs to know.