I manage to get the answers from this post –> https://wordpress.org/support/topic/change-required-fields-in-registration-form?replies=3
Instead of using ’em_event_validate’
add_filter('em_event_validate','em_validate', 1, 2);
should be using ’em_booking_validate’ in the filter hook
add_filter('em_booking_validate','em_validate', 1, 2);