You should have the option to reply in the "App Review" page on App Store connect. I would simply reply there and state, as you did here, that you are currently only offering a free plan. If you don't need it right now, you could simply remove the "In-App Purchase" capability/entitlement and add it back in a subsequent build when you submit the version with paid plans.
Topic:
App & System Services
SubTopic:
StoreKit
Tags: