Mongoose is the error message you will get when matchmaking couldn't find other players and eventually timed out. If you're loading into public spaces alone and can't matchmake in any of the activities then you potentially have some network issues. The first thing to try is clearing your console and network caches. You do this by unplugging your console, router and Internet Modem/Gateway from power. Leave all these devices disconnected from power for at least 5 minutes to fully clear their respective caches. After 5 minutes reconnect power, start up your network, then start up your console and try the game again. If clearing caches doesn't resolve the issue then you'll need to read through Bungie's [url=https://help.bungie.net/hc/en-us/articles/360049496531-Network-Troubleshooting-Guide]Network Troubleshooting Guide[/url].