That's not the usual error message you'd get if the token didn't work.

Are you sure that's the message that pops up after your client clicks on accept or once he's directed to the website?

See this article for how the invitation acceptance process works on your client's side and check with your client at which stage he encountered that problem: https://proconnect.intuit.com/support/en-us/help-article/accounting-bookkeeping/accept-link-invitati...

On your dashboard, does it show whether the invitation has been accepted?