Export from Oracle 10g and import to Oracle 12c unable to create users
You have created a CDB (Container Database), and connected to the root container. There you are not allowed to create regular users, you can create common users whose name starts with C##.

If you do not care about or want to use pluggable databases, I suggest that you drop your empty 12c database, and create a new non-CDB with DBCA (Database Configuration Assistant).