# The error `env.os`

**environment**: something the run needs is missing or different where it runs (a package, a file, a device, the network): the error names what to install, fetch or attach.

Where MIME gives it, with what its message says happened and what its action says to do (`{…}` is filled in from the experiment):

- in `mime/env/_check.py`:
  - what happened: `{…} is {…}; the base image {…} holds {…}`
  - what to do: `run in {…}, or check without --base`

Reading an error and reaching its full traceback: [reading an error](../reading_errors.md). Every code: [the index](index.md).