SCOTT@orcl_12.1.0.2.0> var muser VARCHAR2(10)
SCOTT@orcl_12.1.0.2.0> ACCEPT muser CHAR PROMPT 'Give username :'
Give username :Test123
SCOTT@orcl_12.1.0.2.0> DECLARE
  2    v_user  VARCHAR2(10) := UPPER('&&muser');
  3  BEGIN
  4    EXECUTE IMMEDIATE
  5  	 'CREATE USER "' || v_user || '" IDENTIFIED BY "Test123"';
  6    EXECUTE IMMEDIATE
  7  	 'GRANT CONNECT TO "' || v_user || '"';
  8  END;
  9  /

PL/SQL procedure successfully completed.

SCOTT@orcl_12.1.0.2.0> SELECT username FROM all_users WHERE UPPER(username) = 'TEST123'
  2  /

USERNAME
--------------------------------------------------------------------------------
TEST123

1 row selected.

SCOTT@orcl_12.1.0.2.0> COLUMN muser NEW_VALUE v_user NOPRINT
SCOTT@orcl_12.1.0.2.0> ACCEPT muser CHAR PROMPT 'Give username :'
Give username :Test123
SCOTT@orcl_12.1.0.2.0> SELECT UPPER('&&muser') AS muser FROM DUAL
  2  /




1 row selected.

SCOTT@orcl_12.1.0.2.0> CREATE USER "&v_user" IDENTIFIED BY "Test123"
  2  /

User created.

SCOTT@orcl_12.1.0.2.0> GRANT "CONNECT" TO "&v_user"
  2  /

Grant succeeded.

SCOTT@orcl_12.1.0.2.0> SELECT username FROM all_users WHERE UPPER(username) = 'TEST123'
  2  /

USERNAME
--------------------------------------------------------------------------------
TEST123

1 row selected.

SQL> select username from all_users where username='TEST123';

no rows selected

SQL> create user test123 identified by "test123";

User created.

SQL> select username from all_users where username='TEST123';
USERNAME
------------------------------
TEST123

1 row selected.