How to check git porcelain on CI step?

I added this step to CI

-yaml
  check_if_there_is_change:
      - run: 
          name: Check that no git-tracked files were modified
          command: |
          FILES_MODIFIED="$(git status --porcelain)"
        if [[ -n "$FILES_MODIFIED" ]]; then
          echo "The following files were modified or added during the build process:"
          echo "$FILES_MODIFIED"
          echo "This will likely prevent successful publishing. Please run the build locally and include these changes in your pull request)"
          exit 1
        fi

But it fails

-plaintext
yaml.scanner.ScannerError: while scanning a simple key
  in ".circleci/main.yml", line 638, column 11
could not find expected ':'
  in ".circleci/main.yml", line 639, column 9

IntelliJ shows invalid child element in block mapping. How to change this? I want to know if files have been modified.