This is the commonest thing that goes wrong with an invitation, and it is the least serious. Somebody meant to accept, life happened, and the window closed.

Open your Circle and find them under Waiting to join. Their row has the two controls you need.

Send it again re-sends the same invitation to the same address, with a fresh window. Use this when you are confident the address is right and it simply got lost or forgotten.

Fix the address lets you correct a typo and send to the right place, without losing the seat you are holding for them. Use this when you look at the row and realise the email has a letter missing.

Either way the new invitation replaces the old one. There is nothing to undo first.

That is the same thing wearing a blunter face: the link they opened is expired or has already been used. Ask them to open the most recent invitation you sent. If that still will not open, send it again from their row.

Try the private link instead of the email. Some invitations land in a spam folder and are never seen, and a link you send yourself, in a text they are expecting, tends to get opened.

The two work differently in a way worth reading once: What is the difference between an email invitation and a private link?