The error 'cost model not set' occurs when using multiple resources in skypilot. This is a known issue in skypilot version 0.4.1. To resolve this, you can either use the latest master branch of skypilot or install the nightly build using the command pip install -U 'skypilot-nightly[all]'. If you prefer to wait for the next release, it is expected to be available in a couple of weeks from now.