PL/SQlL All the rows that have only one of the same value in as certain field

Hello expert,

Need to find the rows in a table that only have a value once in the
 parent_company_id field.

Tried
SELECT employer_id,
   employer_name,
  employer_category_code,
   parent_company_id,
  alternate_name
from ccm_employer
GROUP BY  parent_company_id, employer_id
HAVING COUNT(*) > 1

But it returns

ORA-00979: not a GROUP BY expression
00979. 00000 -  "not a GROUP BY expression"
*Cause:    
*Action:
Error at Line: 2 Column: 3

How can I get the needed result?

Thanks.

Allen in Dallas