ORA-01935
missing or invalid user name
Cause
No user name was specified, or the specified user name was invalid. A user name can contain no more than 128 characters and must begin with a letter, and may only consist of alphanumeric characters and the special characters $, _, and #. If the user name contains other characters, it must be enclosed in double quotations marks. The user name may not be a reserved word. An existing user name must be specified when using the ALTER USER and DROP USER statements. Existing user names can be found by querying the DBA_USERS view.
Action
Specify a valid user name.