I'm here to ensure that you'll be able to add a new team member on your QuickBooks.
May I know what specific internal error message did you received? Any further details are much appreciated so we can get this issue sorted out.
For now, you can try accessing your QuickBooks using a private browser. This is to check if the issue has something to do with your browsers' cache. The cache is stored for increased loading performance, but too much can accumulate and bog down processes.
Here are different keyboard shortcuts to open a private browser: