Continue with PayPal immediately direting to checkout_failure.php?

I've set up eCart on the following website, which requires you to complete the form in full (I'm afraid)…
https://www.sportsmedicalcertificates.com/health-screening-form.php?tab1=1

Once you "Add to Cart", after completion of the above form, you are directed to:
https://www.sportsmedicalcertificates.com/health-screening-form-checkout.php

...which is a customised version of checkout.php (which is available on the server at https://www.sportsmedicalcertificates.com/checkout.php).

I ran the checkout wizard, as advised, and entered Sandbox details for now, however when I choose the PayPal option and opt to proceed, it takes me straight to checkout_failure.php.

I'm also having some problems, since I customised the page a bit, when selecting between PayPal or Card payment (the address doesn't display sometimes, and the PayPal button remains in view when it should be being hidden).

Would you be able to get this part of the website working for me Ray, in a Premium Support session?

Firstly, I need to get it all working with the client's Sandbox details so that they can test it.

Upon successful payment, by whichever means, I need to update an order table however, unique to this project, I don't need to save order details to database, prior to payment, because I'm already storing all that via the form that's completed before you get to payment.

I'll put FTP details in a private window below, database connection details etc.
Let me know when you'd be free to help with that.

Thank you.
NJ