Calendar is not visible, and I have Error: jQuery(…).datepick is not a function.

If you have the situation that calendar is not visible in your booking form but all other fields is visible.

And you are have JavaScript error, similar to this:

Error: jQuery("#calendar_booking" + bk_type).datepick is not a function

or this:
TypeError: jQuery(...).datepick is not a function

Reason of this issue is loading of jQuery library 2 times in your website.

The loading process is following:
1) Firstly is loaded the standard jQuery (ver 3.6) from WordPress (and its fine):

<script type='text/javascript' src='../wp-includes/js/jquery/jquery.js?ver=3.6.0' id='jquery-core-js'></script>

2) Then load the scripts of the Booking Calendar, where is defined all functions, including the datepick funcion.
3) Then your theme (or some other plugin) load the jQuery again.
<script type="text/javascript" src="..../wp-content/themes/my_custom_theme/js/jquery.min.js"></script>

This last load of the jQuery is overwrite all previously defined variables and we have this error.

Please open the header.php (or footer.php) or functions.php file of your theme and comment the loading of the jQuery, to prevent load it for the second time.
Otherwise please contact support of your theme (or plugin) relative loading of jQuery library  in correct  way,  relative to  WordPress best  practice and not load it directly in the header or footer files.