When I try to add the product to the cart, it says "Variant can only be purchased with a selling plan." What to do?

This issue happens when you try to add a product to the cart which is marked as subscription only product but your shop tries to add the product to the cart without the selling_plan parameter.
To resolve this, you can either go into Shopify Admin -> Products -> click on a product -> scroll down to the Subscriptions card -> un-check the box in front of the Only show this product with these purchase options label OR you can try the steps listed below:
  • Make sure that you have an auto-charging subscription rule set up for this product.
  • Make sure that the widget shows up on the product page.
  • If the widget doesn't show up on the page, try to install it (instructions are in Installation) section of the Seal Subscriptions app.
  • If the widget shows up, then it could be that your theme just doesn't serialize the whole form when it tries to add the product to the cart. You can contact your theme developers to resolve this issue.