Is the email address you use to send them to invite for Intuit Link the same as what they use for QB?

Did your client click on the button within your invite (which contains a token) and ensure the same email address to which the invite is sent is entered as the User ID (which should be populated automatically by the browser) when logging into Intuit Link for the first time for the tax year?  This is important as this is how the system connects your client's Intuit Link account with yours.