In QuickBooks Online (QBO), you can add up to two accountants, and only the master admin (MA) and accountant users can invite them. Also, ensure that you're using your master administrator's login credentials to do the process.
If you're already logged in as MA and the issues persist, let's perform some troubleshooting steps on the browser. To start, try signing into your QBO using a private browser (incognito).
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Once logged in, invite your accountant again to double-check. If it's successful, return to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.