How to override node add template for custom content type?
This template will be used when a node edit form specifies 'node_edit_form' as its #theme callback. Otherwise, by default, node add/edit forms will be themed by form.html.twig.

Try this on your theme:

use Drupal\Core\Form\FormStateInterface;

function YOURTHEME_form_node_form_alter(&amp;$form, FormStateInterface $form_state) {
  $form['#theme'] = ['node_edit_form'];
}


After this your theme can read template node-edit-form.html.twig. 

https://api.drupal.org/api/drupal/core%21themes%21classy%21templates%21content-edit%21node-edit-form.html.twig/8.2.x