Hi there, @rawvoice.
Allow me to step into this conversation and provide some details about verifying SSN number in QuickBooks Online.
Have you tried checking the vendor's contact list and employees directory to verify if the SSN was used already? If so and the issue persists, I'd suggest reaching out to our Customer Care Team. They have the proper tools to check on your set up and verify the main cause of this issue.
Also, here are some references that you can check out about handling SSN in QuickBooks Online:
This should get you pointed in the right direction.
I want to ensure that this gets taken care of for you, so please feel free to update me by posting a comment below. Have a lovely day!