Here’s a list of consolidated list of things to try if it happens again:
- Ensure the person you are inviting is online
- Close/reopen the game
- Exit the invite menu to the World Map and try again
- If you previously typed in the invite code by hand, copy+paste it from an outside document
- If you previously copy+pasted it, type it by hand
- Invite players from their profile to your Guild
- Have another Guild member invite the player
- Send support the invite code of the person you are trying to invite
