In-app purchase gives developers the flexibility to support a variety of business models in their apps. However, your app unlocks or enables additional functionality with mechanisms other than in-app purchase, which is not appropriate under App Store Review Guideline 3.1.1.
Specifically, your app uses promo codes to unlock courses.
It would be appropriate to remove these features from your app and any other feature that unlocks or enables functionality with mechanisms other than the App Store.
You may also consider making this unlocked content available to your users as in-app purchases.
Learn more about our policies for apps that offer paid digital content and services in App Store Review Guidelines 3.1.1 and 3.1.3.
For details, next steps, and to ask questions about these issues, please visit the App Review page in App Store Connect.
App Store Review