I follow your step and the calendar didn't show up. I've change some of the code in the javascript file to:

$(document).ready(function() {
    $('.calendar').fullCalendar({
   //doing something here
   });
});

The calendar shows up and everything is fine, then I add whatever the remaining code inside and here it is:

$(document).ready(function() {
    $('.calendar').fullCalendar({
     header: {
        left: 'prev,next today',
        center: 'title',
        right: 'month,agendaWeek,agendaDay'
      },
      selectable: true,
      selectHelper: true,
      editable: true,
      eventLimit: true,
      events: '/events.json',
      
      select: function(start, end) {
        $.getScript('/events/new', function() {
          $('#event_date_range').val(moment(start).format("MM/DD/YYYY HH:mm") + ' - ' + moment(end).format("MM/DD/YYYY HH:mm"))
          date_range_picker();
          $('.start_hidden').val(moment(start).format('YYYY-MM-DD HH:mm'));
          $('.end_hidden').val(moment(end).format('YYYY-MM-DD HH:mm'));
        });
        $('.calendar').fullCalendar('unselect');
      },
      eventDrop: function(event, delta, revertFunc) {
        event_data = { 
          event: {
            id: event.id,
            start: event.start.format(),
            end: event.end.format()
          }
        };
        $.ajax({
            url: event.update_url,
            data: event_data,
            type: 'PATCH'
        });
      },
      
      eventClick: function(event, jsEvent, view) {
        $.getScript(event.edit_url, function() {
          $('#event_date_range').val(moment(event.start).format("MM/DD/YYYY HH:mm") + ' - ' + moment(event.end).format("MM/DD/YYYY HH:mm"))
          date_range_picker();
          $('.start_hidden').val(moment(event.start).format('YYYY-MM-DD HH:mm'));
          $('.end_hidden').val(moment(event.end).format('YYYY-MM-DD HH:mm'));
        });
      }
   });
});

But the form doesn't show up and what am I missing?

I didn't receive any Javascript errors in the browser.