Civievent on Wordpress : event registration not possible when multiple user registration to an event

When multiple registration is activated on an event, When we try to register more than one users, the next page ( should be the summary page or some requests about the other users ) is blank.
When we select 1 user participant every think is going well ...

In the form there is an action "<select onchange="allowParticipant()" name="additional_participants" id="additional_participants" class="form-select">"
and this allowParticipant function in the code : "function allowParticipant( ) { }" is empty ...