How can I remove the default value _none in a select list?
First unset the value like below.

unset($form['field_registration_type']['widget']['#options']['_none']);


Then set your drop down value to null, like below.

$form['field_registration_type']['widget']['#options'] = ['' =&gt; 'Select'] + $form['field_registration_type']['widget']['#options'];


Now your drop down value is null and your validation will work as expected.