QuickBooks Online won't accept an employees social security number. I have a copy of their social security card to verify it is the correct.

I can help you with adding your employee to your QuickBooks Online account, @sherryhi92.

First, try to verify your employee's SSN using the Social Security Number Verification Service. Once confirmed, try adding your employee once again.

If you get the same error, try clearing your browser's cache. It stores temporary internet files from your browsing history. However, as it collects too many files, this may cause performance issues in your account.

I've got some steps to help fix this issue. First, you'll have to sign in to your QBO account using a private browser. This helps isolate the issue since it doesn’t store data in the cache. Here are the following keyboard shortcuts: 

• Safari: Command + Shift + N
• Mozilla Firefox: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N
• Microsoft Edge: CTRL + Shift + P

If it works, here's how to clear cache and cookies to fix issues when using QuickBooks Online. Restart your browser right after. You can also try to log in to your account using other supported browsers (Firefox, Google Chrome, Safari, Microsoft Edge) if the steps above didn’t work.

Refer to this article for more information about fixing an employee's Social Security number (SSN): Correct your employee's SSN. 

In case you find some errors on your employee's W-2 or your W-3, check this article for detailed guidance on how to fix them: Fix an incorrect W-2 and W-3. 

You can always go back to this thread if you need further adding your employee's profile in QBO. Keep safe, and have a wonderful day!