When setting up the Yammer "create message" and "create activity" Actions, you're given a change to select which External Network you'd like to publish into.
Although the list appears dynamic, it is actually fixed to the moment in time when your Yammer Account was connected to Zapier. If you have recently been added to a new external network and would like to publish into it, you need to simply re-connect your Account from your settings or in the Zap editor.
Connection reset by peer
These are temporary network hiccups by Yammer's servers. It is usually safe to ignore these (or try again later) as they are very ephemeral.
Your network is not allowed to request an OAuth token for this Application
This error is commonly thrown when the Yammer organization you belong to has explicitly disabled support for third party integrations. You should speak with your administrator on getting this enabled.