Hey, lead developer of Everest here.
Can you please try following the Everest installation instructions? You shouldn't need to compile Celeste.Mod.mm from source, as we've got devbuilds.
Also, this isn't the right place for modding questions / issues. Please join the Discord server and ping me (@0x0ade#1584), mentioning that you're minecraftgamerpc66 on itch.io.