Hello.
You can activate the “auto cancellation” pending bookings that do not have successfully paid status during a specific period of time. You can activate it at  the Booking > Settings General page  in the auto cancellation / approval  section. So  such  bookings will  be moved to  trash,  if they  will  not be paid.

Also please note. Be sure that you have correctly set time-zone at the WordPress > Settings > General page. Otherwise it’s possible that the time of creation of booking was already in the past and you have reached the time limit for auto cancelation of such bookings. Also if you set a low time interval of auto cancelation, like 5 minutes then the client can not be in the time about the payment. Please increase the time of auto cancelation at the Booking > Settings General page. 

If you are using the PayPal standard integration and your visitors is made the payment for the booking, but do not click on the "return link" on PayPal site to your site, so Booking Calendar do not know that the payment is successful - you will see unknown payment status, and this booking can be auto canceled with that feature.  Stripe has to auto redirect to the booking page automatically. 

So sometimes it's possible that the successfully paid bookings will not have this status and that's why it can be auto-canceled, if you do not approve them.
Ideally (for PayPal) you will need to activate and correctly configure the PayPal IPN system, at the Booking > Settings > Payment page and in the PayPal account. So in this case PayPal has to send the messages directly to your booking system. 

The Auto cancelation starts work when someone open the front end side of your website. 
Please open the page with your booking form, and then check your booking at the Booking Listing page, relative Auto cancelation. 

Please note, auto-cancelation can execute every  1 hour (or other period),  if you have configured CRON at  your website or if some visitor is visited your website,  and cancelation  time exceed that  specific "auto  cancelation " time duration from  last  cancelation . 

If nobody  visit website during "1 hour" (or other period of time),  so  system  just  does not start script for initializing  checking about  (auto  cancellation,  etc... ) some bookings. In this case,  you need to  configure CRON script at your server. Currently  its working with  pseudo-cron functionality:

 WordPress Cron is what many people refer to as a “pseudo-cron system.” The difference is in how UNIX cron and WordPress Cron take action. A typical UNIX cron system runs in this order:

- A time tied to an action occurs.
- Cron runs the action tied to that time.

With WordPress Cron, it works a little differently:

- A visitor comes to any page on your WordPress website.
- WordPress Cron checks each cron event to see whether the scheduled time has passed.
- If the scheduled time for that event has passed, then WordPress Cron executes any actions tied to that event.

Please check more here : https://wpbookingcalendar.com/faq/cron-script/ 

Hello.
Please open this file https://www.lescocottes-terresainte.com/wp-content/plugins/header-footer-elementor/assets/css/header-footer-elementor.css

( you can check how to edit files in WordPress menu in this article https://wpbookingcalendar.com/faq/how-edit-file-in-wp-menu/ )

then find this code:

.ehf-header #masthead { z-index: 99; position: relative; }

and replace it to this code:

.ehf-header #masthead { z-index: 9999; position: relative; }

Please do not forget to clear browser cache before testing these changes.

Disclaimer. Unfortunately we can not start, right now, some personal customization or custom development, because we have almost no free time.
Check more about, what support we are providing here https://wpbookingcalendar.com/faq/what-support-do-you-provide/
Thank you for understanding.
Please note, if you modify the source code of the Booking Calendar, we will not guarantee the correct work of the plugin and do not support it.