Secret key for Maian Checkout

Hello Ian

How are you? Remember me, Billy Joe Conor? Years ago I bought Maian music and you helped me solve an issue with customizing it for my website. Since then, my songs are on Apple Music, CD Baby and at all other online vendors. Because of that I stopped selling them on my website.

I'm currently building a website for a musician friend who wants to teach piano online. Since he also wants to sell keyboards and accessories on his website, onlinepianoteacher.org , I picked Checkout for him to sell a small number of items. Just now I'm installing it, but nowhere on your website can I find where to generate the secret key for it? Would you please let me know where?

Regards,

Billy Joe

EDIT: Hello again Ian. As soon as I finished making the above post I realized that the secret key would be determined by me. However, my initial understanding was that I needed to generate one on your website.

Thank you. Be well,

Billy Joe

EDIT-2:

Hello again Ian:

I was able to install Checkout. However, when an item is added to basket, it isn't actually added, as zero items show at checkout. This happens as well when the item is added to basket from the product page.
What's causing it?
Site:
https://www.onlinepianoteacher.org/2checkout/index.php