Calendar is not visible. What is wrong ? (version 4.0)

The reason of this issue can be any JavaScript errors at your site. Probably some other active plugin or actual theme is generate some JavaScript error, because of that the calendar will not show up. You are need to fix this error(s).
Firstly try to find what plugin or may be theme is generate this issue.
- Try to deactivate one by one all actual plugins to check if the calendar will show up or not, to find what plugin is generate this issue.
- If the previous do not help, try to deactivate the actual theme and activate the default WordPress theme, then test it, if the reason of issue in the actual theme.
- If the previous steps do not help, and you can not solve the issue, please check the description of the JavaScript error, and send it with link to the page, where this issue is exist. so then we will try to help you.

Some other possible issues.

I am get this JavaScript error:

Error: a("body").on is not a function
Source: .../booking/interface/bs/js/bs.min.js
Line: 7

Solution:
Its seems, that this issue is because your theme is used the old version of jQuery.
Please, download the latest version of jQuery and integrate it into your actual theme or just use the latest version of jQuery from WordPress installation (just comment loading of the jQuery from the theme and the Booking calendar will add load of the jQuery from your wordpress installation (of course, recheck that this installation also have to be up to date)).

If you are get the JavaScript error similar to this:

Error: jQuery("#calendar_booking" + bk_type).datepick is not a function
Source: .../wp-content/plugins/booking/js/wpdev.bk.js
Line: 161

So then please check the troubleshooting instruction here.