InvalidArgumentException: "Class "\Drupal\environment_indicator\Form\EnvironmentIndicatorSettingsForm" does not exist."

OK, just realised that the documentation needs to be updated for this to not happen. The settings.php (or settings.local.php) adjustments must be made first, and for D10, the variable is $settings, not $config. That said, it probably shouldn't catastrophically fail if those changes are not yet made, but rather should show instructions to that effect on the configuration page.

Also, not seeing any changes now despite being able to access the configuration page. Will open separate issue for that.

I am unable to reproduce this error, here is my Lando setup. Feel free to reopen if you disagree.

name: environment_indicator
recipe: drupal10
config:
  php: '8.1'
  via: apache:2.4
  webroot: web
  database: mariadb:10.6
  xdebug: false
services:
  database:
    type: compose
    services:
      image: mariadb:10.6
      command: docker-entrypoint.sh mariadbd
      restart: always
      ports:
        - '3306'
      environment:
        MARIADB_ALLOW_EMPTY_ROOT_PASSWORD: 'true'
        MARIADB_DATABASE: drupal10
        MYSQL_DATABASE: drupal10
        MARIADB_USER: drupal10
        MARIADB_PASSWORD: drupal10
  appserver:
    overrides:
      environment:
        SIMPLETEST_DB: 'mysql://drupal10:drupal10@database/drupal10'
        SIMPLETEST_BASE_URL: 'http://appserver'
      volumes:
        # Don't share our host working directory as /app. We want /app empty for composer.
        - /app
        # Instead share our host working directory as a standalone package.
        - .:/usr/local/environment_indicator
    build:
      # Create a new Drupal project and use the module as a non-packagist repository.
      - composer create-project --dev drupal/recommended-project:10.0.x /app
      - composer config extra.enable-patching true
      - composer config extra.composer-exit-on-patch-failure true
      - composer config allow-plugins.cweagans/composer-patches true
      - composer require cweagans/composer-patches
      - composer config minimum-stability dev
      - composer config allow-plugins.phpstan/extension-installer true
      - composer require --dev drupal/core-dev:^10.0 drush/drush phpspec/prophecy-phpunit:* phpstan/extension-installer mglaman/phpstan-drupal phpstan/phpstan-deprecation-rules drupal/config_inspector drupal/devel
      - composer config repositories.localdev path /usr/local/environment_indicator && composer require drupal/environment_indicator:\*@dev
  node:
    type: node:20
    build:
      - yarn install
  chromedriver:
    type: compose
    services:
      image: seleniarm/standalone-chromium:4.1.4-20220429
      command: /opt/bin/entry_point.sh
tooling:
  # Provide a command to install Drupal.
  install:
    service: appserver
    cmd:
      - /app/vendor/bin/drush --root=/app/web site:install --account-mail=noreply@example.com --account-name=admin --account-pass=admin --db-url=mysql://drupal10:drupal10@database:3306/drupal10 -y --verbose
      - /app/vendor/bin/drush --root=/app/web en -y environment_indicator environment_indicator_ui devel config_inspector
      - /app/vendor/bin/drush --root=/app/web uli -l https://environmentindicator.lndo.site/
  # Provide Drush tooling to automatically know the Drupal root.
  drush:
    service: appserver
    cmd: /app/vendor/bin/drush --root=/app/web
  phpcs:
    service: appserver
    cmd: /app/vendor/bin/phpcs -s --colors --standard=Drupal,DrupalPractice --extensions=php,module,inc,install,test,profile,theme,css,info,txt,md,yml web/modules/contrib/environment_indicator
  # Provide PHPCBF tooling to fix coding standards.
  phpcbf:
    service: appserver
    cmd: /app/vendor/bin/phpcbf -s --colors --standard=Drupal,DrupalPractice --extensions=php,module,inc,install,test,profile,theme,css,info,txt,md,yml web/modules/contrib/environment_indicator
  # Provide phpstan tooling to check for code quality and deprecated code.
  phpstan:
    service: appserver
    cmd: /app/vendor/bin/phpstan analyse --configuration web/modules/contrib/environment_indicator/phpstan.neon web/modules/contrib/environment_indicator
  # Provide phpunit tooling to run unit tests.
  phpunit:
    service: appserver
    cmd: /app/vendor/bin/phpunit --configuration /app/web/core/phpunit.xml.dist --bootstrap /app/web/core/tests/bootstrap.php /app/web/modules/contrib/environment_indicator
  yarn:
    service: node
    cmd: yarn
  eslint:
    service: node
    cmd: yarn run eslint --color