You can purchase StrongLifts Pro from settings - account - Pro subscription (or just click here).
If you can't purchase Pro, try this...
- Enable in-app purchases in your phone settings > general > restrictions. See https://support.apple.com/en-us/HT204396
- Try a different Apple ID. If you use a work phone, your company may have blocked purchased. Sign in with a different Apple ID. See https://support.apple.com/en-us/HT204053
- Update your payment method. Apple usually accepts credit cards and gifts cards, but this depends on your country. See https://support.apple.com/en-us/HT202631
- Update your billing information if your payment gets declined. See https://support.apple.com/en-us/HT203005
- Try to restart your device. See https://support.apple.com/en-us/HT201559
- Reinstall the app with your current Apple ID. Login with Google/Facebook first under settings - account, so you don't lose workouts. Then delete the app, and reinstall using this link - https://itunes.apple.com/us/app/stronglifts-5x5-workout/id488580022?mt=8
- If you get "Can't connect to iTunes", see this - https://support.apple.com/en-au/HT201400
Apple processes payments on iPhone/iPad.. We can't accept payments, and can't check your billing info/Apple ID. So if you still can't purchase Pro after trying the above tips, please contact Apple - https://support.apple.com/en-us/HT204084