If you're seeing Error completing purchase when trying to become a Medium member on an iOS device, please make sure that your in-app purchases are enabled on your device to be able to make the purchase.
Turning on in-app purchases on your iOS device
- On your iOS device, open the Settings app and tap Screen Time.
- Tap Content & Privacy Restrictions.
- Tap iTunes & App Store Restrictions.
- Tap In-app Purchases.
- Tap Allow.
Once you do that, go back to the Medium app and try subscribing again.