This is something that happens when the jQuery library you have chosen in your WHMCS Bridge Settings is incorrect; usually if you choose WordPress this may happen.

Log in to your WordPress Admin, go to the WHMCS Bridge Settings page and change the "jQuery Library" drop down menu from WordPress to WHMCS (or if you already had it on WHMCS, then change it to WordPress), click on "Save Settings" and then try again.

Note: Clear your cache and do a full page refresh before testing again.