I’ve read through other questions regarding the same error message. They all seem to end up being due to typos or the file path. I don’t believe that is the case here, but we’ll see. I have a working instance of the same code aside from the names, which is all the more confusing.

I enable the module below. Select place its block. And then when I hit the page, receive:

InvalidArgumentException: The form argument DrupalterminalFormTerminalCliForm is not a valid form. in DrupalCoreFormFormBuilder->getFormId() (line 197 of core/lib/Drupal/Core/Form/FormBuilder.php).

I’ve verified that getFormId receives an empty form_id. I’ve read that this error message indicates not a bad form in those specific cases but a form file that cannot be found. I’m not certain which is the case, here.

/modules/custom/terminal/src/Form/TerminalCliForm

namespace DrupalterminalForm;

use DrupalCoreFormFormBase;
use DrupalCoreFormFormStateInterface;

/**
 * Class TerminalCliForm.
*/
class TerminalCliForm extends FormBase {

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

  /**
   * {@inheritdoc}
  */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#attached']['library'][] = 'terminal/terminal';
    $form['canvas'] = [
      '#type' => 'textarea',
    ];
    $form['commandline'] = [
      '#type' => 'textfield',
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
  */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    // Nothing.
  }

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

}

/modules/custom/terminal/src/Plugin/Block/TerminalCliBlock – I’ve tried this with and without the use statement for the form class. In the other module that works correctly, it is present.

    namespace DrupalterminalPluginBlock;

    use DrupalCoreBlockBlockBase;
    use DrupalterminalFormTerminalCliForm;

    /**
     * Provides a 'CliBlock' block.
     *
     * @Block(
     *   id = "terminal_cli_block",
     *   admin_label = @Translation("CLI block"),
     *   category = @Translation("Engine block")
     * )
    */

    class TerminalCliBlock extends BlockBase {

     /**
      * {@inheritdoc}
     */
     public function build() {

       $form = Drupal::formBuilder()->getForm(TerminalCliForm::class);

       return $form;
     }
   }