The error may occur while trying to login to your oscommerce admin panel.
1. Open your database management tool (PhpMyAdmin or similar)
2. Open the SQL tab at the top
3. Into the SQL query window paste the following query and press Go button:
4. When the query is complete open your osCommerce admin panel. You’ll be notified that there are no administrator account. Type your new admin username and password.
Now the admin account has been successfully created. Type the admin details again to enter the admin panel
Quoted from : Template Monster