To assign multiple class codes to an employee, ensure your class setup is configured as One to each row in transaction, @Vertigo2.
Here's how to check your class setup:
- Click the Gear icon and select Company settings.
- Go to the Advanced tab and choose Categories.
- In the Assign class dropdown, check if it is One to each row in transaction.
- If you made changes, click Save and then Done.

After that, you can now assign classes to your employee in payroll. Here's how:
- From Employees, click Run payroll.
- Select a pay schedule.
- In the Action column, tick Edit classes & projects.

We're always here if you need further assistance in QBO. I'll keep this thread open for your future reference.