Goodly

When you see "Object reference not set to an instance of an object", that means it's trying to get the value of a variable in your code, but because that variable is null, it fails to find anything.
The issue is that PlayerAwards.Get(p.name) can give you either:
List<string>, a list of 0 or more awards that the player has got before
null, if the player has never gotten an award before
You cannot call .CaselessContains on a null variable because that function requires a valid string to run. The solution is not to call PlayerAwards.Get and CaselessContains in the same line, so that we can check if the list is null first.
By the way, please make sure you indent the code (tab or four spaces) every time there is an open bracket {, otherwise the code will become extremely difficult to understand.
An example of fixes is attached.
CmdGiveInsane.cs

If you want to play multiplayer LAN, you will need to run your own server. https://github.com/ClassiCube/MCGalaxy?tab=readme-ov-file#setup
Once it's running and you can join it, you can check the wiki for how to do basic things like create new levels. https://github.com/ClassiCube/MCGalaxy/wiki
Once your server is up and running, you can import levels from other multiplayer servers by saving the level clientside (Save level... menu option), then moving that level into the server's extra/import folder and using /import [mapname]

However it should be noted that if the level you saved has any kind of special gameplay or dialogue, you cannot preserve that by saving it for yourself. It can only be experienced on the server you found it on.

Update to the latest dev build using /update latest

That should fix no errors being printed when you use /pcompile


I don't know anything about compiling with Ubuntu, but it looks like you added a space between -r: and the _extralevelprops.dll file which isn't how you formatted the MCGalaxy one. Try removing that space?

Because the server is setup in an attention seeking and mysterious way.
Disconnect message is "Skew issue" rather than something clear like "This server is private". This can make players wonder if they've been personally excluded rather than knowing it's a whitelist
The name of the server keeps being changed to parody other servers, further bringing attention from any players who are regulars of the parodied servers
It often has a significant amount of players online compared to the average server, prompting further questions about what's going on there

Uninstall the SteamRichPresence plugin.

Try going to the hotkeys menu to remove it (rather than controls menu)

Grief and destroying levels is not typically considered a violation of the TOS. If you don't want your maps to be destroyed, I recommend that you avoid servers that have no rules (anarchy) such as the free owner one.