## TL;DR
Fix the YAML syntax error in mkdocs.yml, usually bad indentation or a stray tab. YAML never allows tabs for indentation, and one misaligned key breaks the whole file.

## Error
```text
ValueError: Config file is not valid YAML
```

## Steps
1. Get the exact line: run `python -c "import yaml; yaml.safe_load(open('mkdocs.yml'))"` and read the marked line and column. Expected: a precise location instead of guessing.
2. Open mkdocs.yml at that line and check for tabs, mixed indentation, or an unquoted value containing a colon (e.g. a URL or time). Expected: you spot the syntax violation.
3. Fix it: replace tabs with spaces, align sibling keys to the same column, and quote values containing special characters like `": "` or leading `*`. Expected: the file uses spaces consistently.
4. Re-run the YAML parse check, then `mkdocs build`. Expected: parsing succeeds and the build proceeds.

## When to use
- MkDocs reports the config file is not valid YAML
- A docs agent or script generated mkdocs.yml
- The config was edited by hand and the build broke

## When not to use
- YAML parses but a required key is missing (different error, add the key)
- A plugin rejects its own section (the YAML is fine; the plugin config is wrong)
- The file is valid YAML but MkDocs warns about unknown keys (harmless)

## Tool compatibility
MkDocs 1.4 through 1.6 with PyYAML. The same rules apply to any YAML config in the docs toolchain.

### Variant phrasings
### YAML error after adding a nav section
Nav entries are lists of single-key maps; a missing dash or wrong indent under nav is the usual culprit.

### "found character TAB that cannot start any token"
The literal tab error. Convert the file's indentation to spaces throughout.

## Why it happens
YAML is indentation-sensitive and tab-hostile, and MkDocs gives only a generic message. Hand edits and generated configs routinely mix tabs and spaces or misalign a nested key, and the parser rejects the entire file instead of the one line.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_dxGslwi-mfA8-T8OiO9kyA