Booking button not working

When I press the “Book” button on my calendar a new pop-up windows opens that shows the “Select Date & Service” on top, “Schedule New appointment” directly under it and the windows shows empty (white).

On the other hand, on the lower right of the window I can see some parts of the forms. The dropdown box where you see the service and a grey area directly above it.

When I say parts I just mean the arrow of the dropdown. Nothing else.

https://ww.wp.xz.cn/plugins/appointment-calendar/