Search notes:

ORA-65096: invalid common user or role name

$ sqlplus / as sysdba

SQL> create user tq84_ora_65096_demo identified by aWeakPassword;
--
-- ORA-65096: invalid common user or role name
--

Check to which container we're connected:

SQL> select
   ses.con_id,
   case when con.con_id = ses.con_id then 'X' end cur_con, -- current container
   con.name
from
   v$session    ses   cross join
   v$containers con
where
   ses.sid = sys_context('userenv', 'sid') -- Identify own session
; 
--
-- CON_ID C NAME
-- ------ - ------------
--      1 X CDB$ROOT
--          PDB$SEED
--          PDBTQ84

We're connected to the root container (CDB$ROOT).

Trying again to create a user (without c## prefix) on a PDB:

SQL> connect sys/elCarosSecret4@pdbtq84 as sysdba

SQL> select
   ses.con_id,
   case when con.con_id = ses.con_id then 'X' end cur_con, -- current container
   con.name
from
   v$session    ses   cross join
   v$containers con
where
   ses.sid = sys_context('userenv', 'sid');
--
-- CON_ID C NAME
-- ------ - ----------------
--      3 X PDBTQ84

We're on a PDB (which is the only visible container now) and ii's now possible to create the user:

SQL> create user tq84_ora_65096_demo identified by aWeakPassword;
--
-- User created.

ORA-65096: invalid common user or role name

See also