"Denial reason code 9" when trying to add a friend to a party in Fortnite

If you get the "Denial reason code 9" error when trying to add a friend to your party, this means that your party is playing on a different platform and at least one of them has crossplay turned off.

Make sure that everyone in your party has crossplay turned on. For details on how to turn on crossplay, check out this article: How do I turn Fortnite crossplay On or Off from my console?