Hopping onto a Fortnite match is a no-effort process unless a pesky error decides to cross your path. While Epic Games generally does a decent job of patching such errors, they can still appear out of nowhere, especially after huge content releases.

The Denial Reason Code 18 can appear when players try to join a party together. When this error pops up, players will continue to stay on the main menu by themselves, unable to join the party of their choice.

What does the denial reason code 18 error mean in Fortnite?

The denial reason code 18 error means the party you were invited to might have already joined a match.

It can also be prompted by other reasons like party-related bugs or when someone joins as the final party member before you accept an invitation.

How to fix the denial reason code 18 error in Fortnite

Wait for party members to get out of their match.
Invite your friends to your party instead of joining theirs.
Restart Fortnite.
Change your Party Privacy mode settings to Public.
Make your Fortnite Party Public, or ask your friends to make theirs Public.
Add your squad mates to your Epic Games accounts as friends.

In general, the denial reason code 18 should be a one-off and disappear on its own as you get a second invite. If the error remains, one of the steps above should be able to fix it. Players looking to help out Epic can also submit a support ticket so the developer can take the necessary steps toward implementing a permanent fix.