Calendar is not visible, but booking form fields are showing?

You can have this issue because of having some JavaScript error(s) at your website.
Probably some other plugin or actual theme generate JavaScript error. That's why the calendar do not show.

To recheck your page for JavaScript errors, open the browser console (usually to see JavaScript error(s) press Ctrl + Shift + J, check more here ).

If you will see the JavaScript error, you can see in description of the error, what plugin (or theme) generate this error. Otherwise you can check for some other known JavaScript errors in other other troubleshooting instructions.

You can also try to check about some conflict with some active plugin or actual theme.
Deactivate one by one all your active plugins at the WordPress > Plugins menu and test about the issue during each deactivation.
If its not help, please deactivate your active theme and active the default WordPress theme.