How to fix ERROR: yaml.scanner.ScannerError: while scanning a simple key in Docker

The error message includes the exact location of the problem. Look at the line and column numbers reported:

ERROR: yaml.scanner.ScannerError: while scanning a simple key
  in "./docker-compose.yml", line 15, column 3

Open your docker-compose.yml file and navigate to that specific line to inspect the formatting around it.

YAML does not allow tab characters for indentation. Convert all tabs to spaces (typically 2 spaces per indent level).

In VS Code:
1. Press Ctrl+Shift+P (or Cmd+Shift+P on Mac)
2. Type "Convert Indentation to Spaces"
3. Select the option to convert

Using sed on Linux/Mac:

sed -i 's/	/  /g' docker-compose.yml

Verify no tabs remain:

grep -P '	' docker-compose.yml

If this command returns nothing, your file has no tab characters.

Use an online YAML validator to identify syntax errors:

1. Go to [YAML Lint](https://www.yamllint.com/) or [Code Beautify YAML Validator](https://codebeautify.org/yaml-validator)
2. Paste your docker-compose.yml content
3. The validator will highlight exact syntax errors

Alternatively, use the docker compose config command to validate:

docker compose config

This parses the file and outputs the resolved configuration, or shows detailed error messages if parsing fails.

YAML requires a space after colons in key-value pairs. Check for lines like:

# Wrong - missing space after colon
restart:always
ports:
  -"80:80"

# Correct - space after colon
restart: always
ports:
  - "80:80"

Pay special attention to any lines you recently edited.

Ensure consistent indentation throughout the file. All items at the same level must have identical indentation:

# Wrong - inconsistent indentation
services:
  web:
    image: nginx
     ports:    # Extra space causes error
      - "80:80"

# Correct - consistent 2-space indentation
services:
  web:
    image: nginx
    ports:
      - "80:80"

The services, volumes, and networks top-level keys should have no indentation. Their children should be indented by 2 spaces.

If you copied your docker-compose.yml from a PDF, Slack message, or website, hidden characters may have been introduced. The safest fix is to manually re-type the problematic section.

Alternatively, paste the content into a plain text editor first, then copy it again to your docker-compose.yml file.

Common sources of hidden characters:
- PDF documentation (often introduces special quote characters)
- Slack or Teams messages (may convert quotes to "smart quotes")
- Web pages with rich text formatting

Prevent future errors by installing a YAML linting extension in your editor:

VS Code:
- Install the "YAML" extension by Red Hat
- It provides real-time syntax highlighting and error detection

Enable the extension for Docker Compose files by adding to your VS Code settings:

{
  "yaml.schemas": {
    "https://raw.githubusercontent.com/compose-spec/compose-spec/master/schema/compose-spec.json": "docker-compose*.yml"
  }
}

This provides autocomplete and validation specific to Docker Compose syntax.