Custom form, add admin editable instructions from settings area to form

I have a custom form that is working great. I'd like to add a field "instructions" right under the title, above the first form field. This "instructions" field is editable in the module settings area. I have tried to get this data like:

$config = $this->config('myform.settings');
$instructions = $config->getValue('instructions');

however, it throws an error:

Call to undefined method Drupal\Core\Config\ImmutableConfig::getValue()

How do I pass the data in a textfield to the form the users are filling out on render?

The method to get a value from an immutable configuration object or an editable configuration object is get().

In a form class that extends ConfigFormBase, the code to use for your case is the following one.

$instructions = $this->config('myform.settings')
  ->get('instructions');

getValue() is a FormState method, which is used to get a value submitted in the form. See AccountSettingsForm::submitForm() as example of how that method is used.

$this->config('user.settings')
  ->set('anonymous', $form_state
  ->getValue('anonymous'))
  ->set('register', $form_state->getValue('user_register'))
  ->set('password_strength', $form_state->getValue('user_password_strength'))
  ->set('verify_mail', $form_state->getValue('user_email_verification'))
  ->set('cancel_method', $form_state->getValue('user_cancel_method'))
  ->set('notify.status_activated', $form_state->getValue('user_mail_status_activated_notify'))
  ->set('notify.status_blocked', $form_state->getValue('user_mail_status_blocked_notify'))
  ->set('notify.status_canceled', $form_state->getValue('user_mail_status_canceled_notify'))
  ->save();