Sorry for the delay, I'm currently working on it, after some tests, it seems like the problem is coming from your "Subscription level" option that you are using through your product page, I tested it with the "Subscription level" set to none and it was working fine with 1 product.
Can you give me more information about how did you set this option ?
EDIT
The problem is coming from the fact that you have set "recurring" subscription for your products, and the only payment method which can handle recurring payment is the
Hikashop Paypal Recurring Payment Plugin as it's said on the
documentation
:
Then, if need recurring payments support, download the HikaShop PayPal recurring payment plugin