Bo3 custom zombies, unable to play with friend

Hello , any opinion would be appreciated . Me and my friend both have t7patch , when we try to join eachother an error gets and says “game lobby not available” my nat type is open his is moderate , what we have to do to play some custom maps together ?

"As of recently, the game will crash in online/private matches; but checking the repo, the game has not been updated for a while, so this seems to be a steam client issue.

If you are experiencing issues with playing in private zombie matches, when the game crashes upon starting; switch your steam version to the beta client. https://help.steampowered.com/en/faqs/view/276C-85A0-C531-AFA3

Switching my game to the Steam beta client fixed the issue immediately."

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