That's weird, TonyPham.

Let's use a private window to check if the error is browser-related. Just press Ctrl + Shift + N on your keyboard to go incognito.

Now, log back in to QuickBooks Online and go back to the adding fields. If you're able to proceed, let's clear the browser's cache. Check this article for the detailed steps: How do I clear my browser cache and temporary Internet files?

You can also try to use a different browser.

Let me know how it goes so I can assist you further if you need it.