Let me help you add your accountant, sugarandskinspa.
First, we'll want to make sure you still have the option to add another one.
We can add up to two accountants in Simple Start, Essentials, and Plus plans. Meanwhile, three accountants can be added in Advanced plan. You can use this link for reference: How many users can I set up?.
Click the Gear icon in the upper-right corner and select Manage Users.
Go to the Accounting firms tab.
Enter the accountant's email address and click the Invite button.
They'll receive an email invitation with instructions on how to accept it.
If you still have an option to add, but still encounters the same error, let's try basic troubleshooting.
First, let's sign in to your account using a private window by pressing this following key shortcuts:
Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Mozilla Firefox & Internet Explorer).
Command + Shift + N (Safari).
If it works, go back to your regular browser and clear its cache. These caches of data are essentially junk files and can be safely deleted to free up storage space. Otherwise, switch or use a different browser.