You need to edit the wpresidence/user_dashboard_add.php and add these lines:
}else{
$has_errors=true;
$errors[]=__('Please submit a Category for your property','wpestate');

mandatory-field
if(isset($prop_category->term_id)){
}else{
$has_errors=true;
$errors[]=__('Please submit a Category for your property','wpestate');
}

mandatory-fields-edit