Hello,
When you are in the product page, if you click on the "add to cart" button, you have a message :
No subscription data for MyTutor Premium | VWO
Which indicate that HikaSubscription did not found a subscription to upgrade (probably because the customer have several).
In such case, I recommend to use the HikaSerial Subscription page :
YOUR_WEBSITE/component/hikashop/subscription/listing
There, you will see the subscription and via the "actions" button, you can choose to upgrade a specific subscription.
Regards,