This issue may be resolved by one of the following:

1.  Ensure you are using a compatible browser listed in the system requirements.
2.  Clear out your cache and cookies , close your browser and then reopen the browser.
3.  Copy the confirmation URL from the invitation email and paste it in a different browser.
4.  Cancel the invitation and send a new invitation.
5.  Send the invitation to an alternative email.
6.  Ensure the email address being used to sign-in to is the same as the email address the invitation was sent to