It's frustrating, but the missing IAP (in app purchase) section on App Store Connect isn't an error (probably), it's a workflow requirement
Here is a short (in reading time) fix
Prep Your IAP: Ensure your IAP is in "Ready to Submit" status with all details complete, including a review screenshot
Clear the Version Page: On your app's version page, you may need to remove any previously attached build
Link the IAP: The "In-App Purchases and Subscriptions" section should now appear. Use the + button to link the IAP
Upload & Submit: Upload a new build of your app (via Xcode or another whatever you use) that includes the IAP functionality, attach it, and submit the entire version for review
If these steps fail, try messaging Apple Developer Support (they might be able to help)