If you are having this error, you need to add the App Blocks that are needed for the new Selling Plan API from Shopify. Here's how:
1. Add the "App blocks" on your Product Page by going to the "Theme customizer."
2. Go to Products > Default Products.
3. Under Product Information, click on Add Block to add "Pre-Order Now - Popups" and "Pre-order Now - Plans."
4. Proceed to the Cart Page and add "Pre-Order Now - Popups," then save your changes.
If you are unable to add the App Blocks on your product page due to your Theme, all you need to do is disable the Shopify Purchase Options in your Settings, and hit save at the bottom part of the screen.
After saving, click "Delete selling plan with settings."
Head to Products in your Shopify admin, choose the product with the variant error, and ensure that the "Purchase options" is disabled.
Upon completing these steps, the error should be resolved, and the app should operate as intended.