Inline Entity Form validation
I suggest you implement hook_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) function.

    function hook_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
      // Check if the $form_id is your node type add form.
             if ($form_id == 'article') {
                // Add a custom validation function. Example:
                $form['#validate'][] = 'custom_form_test';
              }
    }


Then you need to implement the custom function where you will put your logic.

function custom_form_test(&amp;$form, FormStateInterface $form_state) {
  // Get value of the field which use the inline entity form display.
 $tags_value = $form_state-&gt;getValue('field_tags');

}


See the screenshot of the $form_state dump and also with the field_tags taxonomy term reference values:


So you can access all the values from the inline form display.
For example to access name value:

$name = $tags_value[0]['inline_entity_form']['name'][0]['value'];


Just looping into arrays. Once you get the value that you want you cand restrict finishing the submision of a node by write a message error. Example:

 $form_state-&gt;setErrorByName(
                'field_tags',
                'The name is WRONG!'
              );


Hope to help you!