Welcome to the Community page,
Can we ask which payroll it is you are using, is it standard or advanced?
The way to tell which one you have is when you click on payroll on the main left-hand navigation bar, if you get two other tabs then that is standard if you get no other tabs showing then that is advanced when clicking on the payroll tab.
Let's resolve this error with your employee's pension contribution, @simonb1.
To check further what's causing this error with your employee's pension contribution, I suggest reaching out to our Customer Support. They can get into your account in a secure environment, identify the error you're having and help you resolve it.
Here's how to contact the QuickBooks Online Customer Support team:
For future reference, let me share the Workplace pensions in QuickBooks Online Standard Payroll guide. It provides an overview of the pension plan in QuickBooks.
Let me know if there's anything else that I can help. I'm always here to assist. Take care and have a great rest of the day!