There is a bug in TOAD (14.2) in which the error 'ORA-00904 "ROWID": invalid identifier' is returned whenever one of the following tables is accessed (Data tab) using Scheme Browser or Describe Objects (Data tab):
sys.system_privilege_map
sys.table_privilege_map
sys.user_privilege_map
This error does not occur in SQLPlus.
Cheers,
Russ
On 21c:
While logged in as SYS, I didn't have that problem.
So I granted READ on SYSTEM_PRIVILEGE_MAP to SCOTT.
Then I logged in as SCOTT and I get ORA-00942, even though I made the grant (maybe there is some system privilege overriding?)
Can you give me some details about your env. and user?
I am in a SYSDBA session within a PDB. The database is 19c and the Oracle client is 12.2.0.1.
When I just select one of the tables in the Editor, there is no error:

But, when I describe the table or just access it in Schema browser, whenever the Data tab is opened, an ORA-00904 error occurs on ROWID:
Cheers,
Russ
Reproduced, logged internally, and fixed for 15.1. Thank you for reporting it.