I can not see captcha at the booking form. How to show a captcha ?

1) Go to the general booking settings page and check ON use CAPTCHA check box and save a page.
2) If you use Standard (free) version skip this step, otherwise go to the form fields customization page add captcha shortcode to the customization form:

<p>[captcha]</p>

3) Check write permission at the folder YOUR_WORDPRES_SERVER/wp-content/plugins/booking/js/captcha/tmp/. This folder have to be write-able.

4) Check that your server was configured for using 2.0 GD library.
Reason.
The cause is that you do not have the version 2.0 GD library loaded in PHP which defines this function, which is used for the captcha in this plugin.

Checking
Use this PHP code to view what extensions you have loaded on your hosting account.

print_r(get_loaded_extensions());
print_r(var_dump(gd_info()));

Just create a text file with the these lines above in your public_html folder (or wherever your web root is), name it with a .php extension like "test.php" and then run it by using your web browser like "http://www.yourdomain.com/test.php "

This does not write correct HTML but will show you the info you need. (Just do a View/Source on your browser if you want a nicer looking format.) Look for text like "[some number] => gd". If you don't see this, you do NOT have the GD library loaded.
Next you want to look for the version number. Something like: ["GD Version"]=> string(27) "bundled (2.0.28 compatible)"
This needs to say version 2.0 or later.

If you do not see GD version 2.0, you must get your hosting provider to install or upgrade your system.

LEARN MORE:
Learn more about the GD library:
http://www.php.net/manual/en/ref.image.php

Learn more about this function:
http://www.php.net/manual/en/function.imagecreatetruecolor.php

5) If you are using Cache plugin (like "WP Super Cache" ), please deactivate it or add the exception to the page with booking form to do not cache this page.

4) Finally, please try to deactivate one by one all your active plugins at the WordPress > Plugins menu. Then retest it again.
If its not help, please deactivate your active theme and active the default WordPress theme. And then retest it again. May be there exist some conflict with some active plugin or actual theme and just need to find the reason of that issue.

If you are still will have this issue, please recheck the error.log in your server configurations to be sure in a reason of that issue.

Kind Regards.