For candidates
Check spam and any "promotions" or "other" tab. Invitations come from a no-reply address and some filters hold those on first contact.
If it is genuinely missing, the organisation can resend it. They can also read you the link — an invitation link is not secret from the person it was issued to.
For recruiters
First establish whether it was sent at all, because "not generated" and "generated but not delivered" are different problems with different fixes.
The candidate's timeline shows the invitation and when it went out. If there is no invitation there, the scheduling did not complete — check the schedule result rather than the mail.
If it was sent but not delivered
The usual causes, in the order worth checking:
- A typo in the address. Common, and invisible: the mail is delivered somewhere real.
- Corporate filtering. Some employers quarantine mail from unfamiliar senders. A template with a sentence of context and the organisation's name in the subject fares better than a bare link.
- A full or disabled mailbox.
Resending
Resend from the candidate's record. Resending reuses the existing invitation rather than creating a second interview, so the candidate does not end up with two links and you are not charged twice.
As a workaround
Copy the invitation link and send it through whatever channel you already use with that candidate. It is the same link.