Hi,
If you implement the onAfterCheckoutStep trigger
www.hikashop.com/support/documentation/6...#onAfterCheckoutStep
in your plugin.
There, check if the data is there, and if not set the variable $go_back to true and the user won't be able to go to the next step.
You can also use the $app->enqueueMessage() function of Joomla to display a message to the user.
Now that's in PHP.
If you also want to do a similar check in javascript, you can redefine the window.checkout.onFormSubmit function like that:
window.checkout.onFormSubmit = function(form) {
// your code
// return false if you don't want the form to submit.
}
That will be triggered when you click on the "next" button of each step of your checkout.