This error occurs due to a mismatch in pre-order settings between Shopify and our app.
To resolve this, follow the steps below:
Go to the app settings and disable the checkbox for enabling "Shopify Purchase Options."
Save the settings and return to the checkbox. Click on the line labeled "Delete existing selling plan."
Open the product you are experiencing issues with. Look for the box titled "Purchase options" and click on the checkbox at the end of this box.
This should fix the issue, and the pre-order button should add the product to the cart.
However, if you wish to use the pre-order partial payment function, then please make sure to follow the steps below:
Go to the app settings and enable the Shopify purchase options.
Once done, this will enable the partial payment for the products again.