When Seahub fails to start, it’s usually a communication problem with the database.
Two things come to mind:
1.) Make sure that you have installed all the prerequisites. You posted the right link - please note that Ubuntu 20.04 requires different (apt and pip) packages than Ubuntu 18.04.
2.) You can try and start seahub running ./seahub.sh start-fastcgi. Seafile no longer supports fastcgi, but it may give you some more debug info.