If you've been gifted a promo / offer code (iOS) for three months of free charting with Read Your Body by your educator, please follow on your iPhone the direct URL link that you've been sent in order to redeem it. This link will load a redemption flow in your App Store app.
If the App Store app displays a message saying that you're not eligible for the offer, please contact your educator to make sure that they've given you the correct type of code - it varies depending on whether you're a new or existing app user (new users are only those who have never started a free trial or subscription in the app).
Happy charting!