getFormId() must be compatible with Drupal\Core\Form\FormBuilderInterface::getFormId (Drupal 8)
In my project it works as public function without arguments since drupal beta 1 until today (i've updated today to rc3):

 public function getFormId() {


You only have to add public to your function.

Otherwise your error suggest, that there are the two arguments form and form_state missing, what is also described in the api.drupal.org:

public function FormBuilder::getFormId

But I don't think, that you need these arguments in the definition of the getFormId(). This is probably a false alarm. You only need these arguments, when using the function in the context of formBuilder, when you don't provide the form through your class, for example like this:

this-&gt;formBuilder-&gt;getFormId($form_arg, $form_state)


Edit:
Berdir has the right clue, you have to change this:

wrong: class AddForm implements FormInterface {
right: class AddForm extends FormBase {


and the use statements for this are:

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;


This is a complete example:

&lt;?php
/**
 * @file
 * Contains \Drupal\mymodule\Form\OrderForm
 */

namespace Drupal\mymodule\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements an form.
 */
class OrderForm extends FormBase {

  /**
   * {@inheritdoc}.
   */
  public function getFormId() {
    return 'mymodule_order_form';
  }

  /**
   * {@inheritdoc}.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

  }


   /**
   * {@inheritdoc}
   */
  public function validateForm(array &amp;$form, FormStateInterface $form_state) {

  }


   /**
   * {@inheritdoc}
   */
  public function submitForm(array &amp;$form, FormStateInterface $form_state) {

  }