Fix the issues reported by phpcs

   requirements:
+    # Access Admin Page Permission
     _permission: 'access administration pages'

The warning is the following.

The administration page callback should probably use "administer site configuration" - which implies the user can change something - rather than "access administration pages" which is about viewing but not changing configurations.

Adding a comment before the _permission line does not change the required permission for those routes.

Please review and merge code.

-    ->fields('sub', ['uid', 'subscription_id', 'plan_id', 'status', 'current_term_start', 'current_term_end'])
+    ->fields('sub', [
+      'uid',
+      'subscription_id',
+      'plan_id',
+      'status',
+      'current_term_start',
+      'current_term_end',
+    ])

Code lines are allowed to exceed 80 characters, if they are more readable. The existing code is more readable than the changed code.

     _controller: '\Drupal\integration_chargebee\Controller\PaymentController::success'
     _title: 'Thank you'
   requirements:
-    _access: 'TRUE'
+    _permission: 'access content'

The report does not say that _access: 'TRUE' must be removed from that route definition, but that a comment explaining why _access: 'TRUE' is used must be added.

     _controller: '\Drupal\integration_chargebee\Controller\CheckoutController::plan'
     _title: 'Subscribe Plan'
   requirements:
-    _access: 'TRUE'
-
+    _permission: 'access content'

The same holds true for this route definition.

+   * The ConfigFactory.
+   *
    * @var \Drupal\Core\Config\ConfigFactory

ConfigFactory is misspelled.

+   * @param \Drupal\Core\Config\ConfigFactory $configFactory
+   *   The renderer.

The description is wrong.

+  /**
    * Construct a new AuthController object.
    *
+   * @param \Drupal\Core\Config\ConfigFactory $configFactory

Since that comment is change, also the method description must be changed: The description for a constructor must start with Constructs a new followed by the class name (including its namespace), and end with object.

   /**
-   * integration_chargebee Service.
+   * Integration_chargebee Service.

integration_chargebee is the module machine name and it does not describe this service's purpose.
Service is misspelled, since it must be spelled using lowercase letters.

+  /**
    * Drupal config.
    *
    * @var \Drupal\Core\Config\ConfigFactoryInterface
    */
   protected $configFactory;

Since that documentation comment is changed, also the property description must be changed, as property descriptions must not start with Drupal.

+  /**
+   * The RequestStack service.
+   *
+   * @var Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;

The request stack. is a better description.

    *   The config factory for the form.
+   * @param Symfony\Component\HttpFoundation\RequestStack $requestStack
+   *   The request service.

The request stack. is a better description.

- /**
+  /**
    * Entity manager.
    *
    * @var \Drupal\Core\Entity\EntityTypeManagerInterface
    */
   protected $entityTypeManager;
 
- /**
+  /**
    * Database connection.
    *

Since those comments have been changed, a definite article must be added to both the descriptions.

+  /**
+   * Class Constructor.
    *
    * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
    *   The config factory for the form.
    */
+  public function __construct(ConfigFactoryInterface $config_factory) {

The description for a constructor must start with Constructs a new followed by the class name (including its namespace), and end with object.

+    # Access granted in all circumstances.
     _access: 'TRUE'

That does not explain why.

   /**
+   * The Config Factory.
+   *
    * @var \Drupal\Core\Config\ConfigFactory

Only the first word in the description must be capitalized.

+  /**
+   * Construct a new CheckoutController object.

The class namespace is missing.

 /**
- * An example controller.
+ * An User controller.
  */

The indefinite article to use with user is the other one.

    *   Connection.
    * @param \Drupal\Core\Messenger\Messenger $messenger
    *   Messenger.

Since that documentation comment is changed, the definite article needs to be added to both those descriptions.

 /**
- * Class ChargebeePlanForm.
+ * Class Chargebee Plan Form.

That is still repeating the class name. Adding spaces does not change that.

    * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
    *   The config factory for the form.

Since that documentation comment is changed, that description must be changed too: The config factory. is sufficient.

+  /**
+   * Create tableselect for plans list.
+   */
+  public function integrationChargebeeCreatePlansSelectTable($plans) {

The verb must be declined to the third person singular.

@apaderno, For the permission issue, I will create another issue. Now, I merging this issue.

@apaderno, I am marking fixed this issue. I will create another issue for the permission.