And the version of your ePO is?
The current version is 4.6.4.
May I know have you enabled Mix mode authentication? Assuming YES
You need not to creat any user in Domain.
Assuming you have Microsoft SQL Management studio installed.
Login with windows authentication.
In the right pane expand security Right click Login>new login Name the user and password Select Misx mode authentication>Under roles select Admin and DB creator
Close this window and expand your DB like ePO4-xxxxx>Security>User look for user that you created >Right click and go into properties>See the publicl s set under premission.Close and log out
Check this user by logging in again .
Now on core\config page of ePO change your user with this .
Apply and you are all set.