There are different user types and levels of access in QuickBooks Online (QBO). You'll want to make sure that you are logged in as the Master Admin of the company so you can add your accountant to QBO.
However, if you are the Master Admin of the company and can't access the Manage Users page, let's try logging in to your QBO account using a private browser. This way, we can check if it's a browser data issue.
Press Ctrl + Shift + N on your keyboard for Google Chrome and Mozilla Firefox.
Press Command + Shift + N for Safari.
Once logged in, go back and try to add your accountant again to double-check. For more guidance, feel free to check out this article: Invite or remove an accountant.
Please know that you can always visit our Community forums again if you have other questions with manage users. I am always here ready to help you whenever you reach out. Have a great day and take care always.