Welcome to the Community. I'm here to lend a hand with your concern about adding new users in QuickBooks Online.
You need to have a Master or Company Administrator user type to be able to add, delete, or change a user's access. If you're unable to add one, let's start with performing some browser troubleshooting to get rid of the error message.
For additional insight into managing any user access, such as adding a new one, I'm adding an article that I recommend:
How to add, delete, or change a user's access.
First thing you can do is log into your QuickBooks account using a private browser. I'm adding the shortcut keys below depending on what you're using:
For Chrome, press Ctrl + Shift + N.
For Internet Explorer or Firefox, press Ctrl + Shift + P.
For Safari, hold down Command + Shift + N.
If it works, you can go back to your regular browser and clear its cache. I also recommend trying a different supported browser to help identify if the problem is system or browser related.
Keep in touch with me here should you need further assistance with adding a user. I want to make sure you're taken care of.
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