Hope I'm able to help, Veronica.  Intuit Link is not the most intuit(ive).

Since he got a message along the line of "account not found", that tells me he tried to login to an account that never existed, from the screen that opens after clicking on the button in your invite.

If he's never had an Intuit account (e.g. for TurboTax), he will need to click on the link in tiny prints in blue towards the bottom, which says New to Intuit? Create an account to create a new account.  Just make sure he does so by clicking on the button in your invite and create the account using the same email to which the invite was sent.