In that case, I believe your issue may be related to the WordPress 5.5 update and either your theme or one of your other plugins not being compatible with jQuery 3.x. You can find more info about that here:
https://wptavern.com/major-jquery-changes-on-the-way-for-wordpress-5-5-and-beyond
It’s possible that either your theme or one (or more) of the plugins you are using could have jQuery code that is not compatible with 3.x. And, with WordPress now making that transition in 5.5, the code in those themes/plugins is now starting to break. And, because our plugin requires JavaScript for the booking form and the edit booking functionalities, if you’ve got JavaScript errors on your page due to other plugins or the theme, then it can cause our plugin to not be able to run correctly.
You can try deactivating your other plugins one by one to see which might be causing the JavaScript issue. It’s possible that it could also be the theme.
And, just for reference, we have the latest version of our plugin running fine with WP 5.5 on our demo site here: https://www.fivestarplugins.com/five-star-restaurant-reservations-demo/