The best and easiest way to resolve this issue is to remove the prefix of the username "C##" to create the new user as normal as using "CDB", simply apply the following, then restart the Oracle DB:

ALTER SESSION SET CONTAINER = CDB$ROOT;

ALTER SYSTEM SET COMMON_USER_PREFIX = '' SCOPE=SPFILE;

While the "CDB$ROOT" is the pluggable database name, and "SCOPE=SPFILE" for required restart to have the change affect.