I have discovered that I didn't have an IAP key set up, so I've done that now, but not sure what to do with this key? I'm using Unity for development and Gley Easy IAP for the purchases. The google play store version of the app is working fine :)

I have the same problem! just to clarify, the section which is missing is on the "submit for review" page itself, not the left nav This section "In-App Purchases and Subscriptions" is normally below the "Build" section - it is missing on my page