With default settings, LifterLMS WooCommerce will only enroll students when a WooCommerce order is marked as “Complete.”
Since WooCommerce was, initially, an ecommerce platform built to sell physical goods, WooCommerce orders are not automatically marked as complete unless you have configured your product to be a virtual product.
If your students are not being enrolled automatically after checkout chances are their order is stuck in the “Processing” stage.
We recommend creating your products to be “Virtual” and “Downloadable” products so that WooCommerce will auto-complete your orders.
You can alternatively change this setting by following these steps: https://lifterlms.com/docs/getting-started-with-lifterlms-and-woocommerce/#auto-enroll