That doesn't look like the right National Insurance number. This National Insurance number is already associated with another employee. Take a look and try again.

Hi, Max. Thank you for joining our forum. I'll clarify the reasons behind the error you're experiencing, and I'll provide my recommendations to fully resolved the unexpected behavior.

QuickBooks Online Payroll Core doesn't allow multiple employee profiles to have the same NI number. Let's ensure the number you're entering is not associated to an inactive employee.

1. Navigate to the Payroll menu and head over to the Employees section.
2. Click the Active Employees drop-down and select Inactive.
3. Select an inactive employee.
4. Go to the Employee details and edit the NI number.
5. You will now be able to assign the correct NI number to the active employee's profile.

After following the steps above and you're still experiencing the issue, I recommend contacting HMRC to further investigate what's preventing you to enter the NI number.

Get back to this thread if you have follow up questions or clarifications. Our team is always here to help.