TL;DR: notebook 7 needs the separate jupyter_server package and it is missing. Run `pip install jupyter_server` and notebook launches.

## The error
```text
ModuleNotFoundError: No module named 'jupyter_server'
```

## Fix it
1. Install jupyter_server:
   ```
   pip install jupyter_server
   ```
   Expected: install completes.
2. Verify:
   ```
   python -c "import jupyter_server; print(jupyter_server.__version__)"
   ```
   Expected: prints the version.
3. Launch notebook again. Expected: the server starts instead of the import error.

## When this applies
Use this when `jupyter notebook` (version 7+) fails with the jupyter_server error, common after --no-deps installs.

## When it does not apply
If you are on classic notebook 6.x, jupyter_server is not needed; a different problem is at play. If jupyter_server is installed but the error persists, check your environment.

## Compatibility
notebook 7.x with jupyter_server 2.x.

## Root cause
notebook 7 was rebuilt on top of jupyter_server instead of carrying its own server code. The dependency is declared, but installs that skip dependencies or partial upgrades can leave it out.

## Edge cases
Installing plain `pip install notebook` pulls jupyter_server automatically; the error mostly hits hand-assembled envs. In conda, install notebook from conda-forge to get the matched server.