I've had this issue and I discovered that the e-mail i used to send the client the Intuit Link request was not the same as an e-mail the client already had registered with Intuit. I resent the Intuit Link request to the e-mail the client had registered with Intuit and it resolved the issue. 

If the client has used any other Intuit Account (i.e. QuickBooks, Turbo Tax, etc.) the Intuit Link request should be sent to the same e-mail the client used with those products.