This is just a mod to improve Chest rendering, and it does not need to be applied to the server . So I recommend marking it as a client mod in fabric.mod.json

  "icon": "assets/fastchest/icon.png",

  "environment": "client",
  "entrypoints": {

It seems the field only serves an informational purpose. The mod still "loads" on the server without any complaints or warnings.
I set it to client only in the json now, but I won't make a new build just for that.