Hi. Can you tell us more about the issue please. Are both you and your friend on Epic? Are you forming the party using the Epic clients friend-list or the in-game Friends Menu? Does that make any difference?
When does the error happen; when you form the party or when you try to search for a match? Does the mode you chose play a role at all?
I noticed a couple of reports of this kind and when I form a party Steam+Epic with the in-game Friends menu I do not have any such issues so any more information to try to recreate it would be great.