Thanks for getting back to this thread @newyorktutor. Let me share some details about the usage limits in QuickBooks Online (QBO).

The message that your client received may be related to data issues in their browser. To confirm, they can access their account using a private browsing mode, which doesn't store cookies or browsing history. Also, since your client is using QuickBooks Simple Start, they can invite two accounting firm users.

Here are the shortcut keys:

• Mozilla Firefox and Microsoft Edge: Press Ctrl + Shift + N
• Google Chrome: Press Ctrl + Shift + N
• Safari: Press Command + Shift + N

If their able to send an invitation, return to their browser and clear its cache and cookies to refresh website preferences that may be causing problems when using QuickBooks Online. Otherwise, they use a supported browser.

If they experience the same issue, I'd recommend contacting our QuickBooks Online Support Team. They'll be able to pull up your account in a secure environment, conduct further research, and create an investigation ticket if necessary.

Nevertheless, if you didn't receive the email invitation from your client, they can check this article for future help: What to do if invited users did not receive your email invitation.

Please know that I'm just post away if you need any further assistance recording expense transactions in QBO.  Don't hesitate to reach out. Stay safe.