How to force a field to be mandatory?
Welcome to SE, You can set contact type mandatory by using hook_civicrm_buildForm something like below

function extension_civicrm_buildForm($formName, &amp;$form) {
  if ('CRM_Contact_Form_Contact' == $formName
    &amp;&amp; !($form-&gt;getVar('_action') &amp; CRM_Core_Action::DELETE)
  ) {
    $form-&gt;addRule('contact_sub_type',  ts('This field is required.'), 'required');
  }
}


HTH

Pradeep