The module settings are specified, but the products for pre order are not displayed in the cart or not available for purchase.

The product’s Availability is Out of Stock.

Solution:

1. Turn off the option that automatically enables the Out of Stock status for products that reach a particular quantity limit.
2.  One of possible solutions is to reconfigure preorder settings. For instance, you can enable the preorder option when the product’s quantity reaches a particular limit but still is On Stock.