Why won't payroll set up wont take the social security number?

Maybe it's not a valid SSN for employment, or is an ITIN?  Does the Employee's Social Security card actually say it's an SSN?

Does it have any restrictions printed on it, like "NOT VALID FOR EMPLOYMENT"?

See:

https://www.ssa.gov/ssnumber/cards.htm

If the card looks normal, check the first two numbers of the SSN. Are they in range for an SSN, not an ITIN?

From https://www.taxact.com/support/[removed]/ssn-itin-atin-valid-ranges :

Thank you for visiting the QuickBooks Community, dpenn409. I'll share details about payroll set up won't take the SSN in QuickBooks Online, then ensure you'll be pointed to the right support team to assist you further on this matter.

The problem of being unable to save the social security number or it says you've entered the incorrect SSN is tagged as an ongoing issue. Our Product engineers are now working to fix this as soon as possible. For now, we're unable to provide an exact workaround to be performed.

I suggest contacting our QuickBooks Online Payroll Support Team so they can add you to our notification list. This will help our engineers determine the number of affected users. Then, you'll receive updates through email once this has been resolved.

Lastly, I'm adding this article so you can view different details on how to complete payroll setup so you can pay your employees smoothly: Get started with QuickBooks Online Payroll.

I'll be more than happy to help if you need any further assistance with your payroll. Just click the Reply button below and I'll make sure to get back as soon as possible. Have a great day, dpenn409.