Just a hunch... and it's more common than you think.  Your client may not have bookmarked the URL in your invitation email, clicked on the hyperlink in your invite, or may even have used a different email address than the one you sent the invite to if they have multiple Intuit accounts.

If they did a search on the Internet, they'd find only the URL (http://accounts.intuit.com/) which ProConnect Tax stopped using in 2017 (although that continued to be the portal for Lacerte/PS). And if they log into Intuit Link using that URL (instead of https://prolink.intuit.com), they'll be able to get into the old portal (because their account credentials are the same for all Intuit products) but see that there's nothing to do.  Same if they have multiple Intuit accounts and log in with an email address that's different from the one used with your invite since Intuit links your account with your client's based on the email address you use in the invite and contingent on them clicking on the hyperlink in your invite, which contains a token, to accept the linkage.

Hope this makes sense.