Sometimes, this happened because the individual has more than one Intuit account and did not use the email address you have to login, causing Intuit Link to open the account that's not linked to yours.

Sometimes, this happens because the token embedded in the invite is not used to access Intuit Link.

Before looking at other possibilities, you should suggest that your client clicks on that link in your invite to open Intuit Link and make sure she logs in using that email address to which you had sent the invite.