If your invitation email has expired or is no longer valid, you will receive the following message when trying to accept the invitation:
"Your invitation has expired or is invalid. Please contact your administrator"
Expired invitation
Invitation emails are valid for 2 weeks from the time they are sent. Check the date and time that the email was received. If it is more than 2 weeks ago, then your invitation has expired.
Invalid Invitation
Only the most recent invitation email is valid. So, if you have received more than one invitation email, you need to use the last one that you received. If you try to use an older invitation, it will not work.
What to do
If you are encountering either of these issues, you have the following options:
Contact your training provider and ask them to send you a new invitation email
Follow the process for Resetting your password and confirm your account that way
If your training provider allows it, you could try confirming your account via Alternative Sign in Method