Please confirm that the email you use to send to your client is the same email they use to sign in. 

It's likely they're signing in with a different email address from the one you used for them. They must sign in with the email address associated with the Link. Once they're logged in, they either need to change their Intuit Account email address to match the one you have or you'll have to un-invite the email you have for them and re-invite them with the email address they're using for their Intuit account.