Recently Pugstorm released a mono version of Core Keeper. You might need it to reference it's assemblies. Here is how to get it.
This guide was made for IL2CPP version of Core Keeper and no longer applies. Normal build of the game already uses mono
Right-click the game's name in Steam and select Properties from the menu
Select Betas
Enter the following password: monobuildsmightnotbeuptodate
Click Check Code
Click the dropdown menu, select the mono branch, and click opt in
Wait for the game to automatically update (or manually update it yourself)
If you wish to have both the mono build and il2cpp one use steamcmd to install and update the mono build. Follow this guide, replacing values for Core Keeper. Core Keeper appId is 1621690