Error 20008
If you get error message 20008, you most likely have an active Yousician subscription linked to your Apple ID on another Yousician account. If you have premium access on one device but not another, go to the Settings → Account tab in the app and check that you are logged in on the premium account on both devices. If that doesn't work, contact our support team.
Error 20004
If you get error message 20004, check that you have enabled in-app purchases on your device. If you're reading this from your iOS device, select this link to edit your payment methods.
Check these instructions from Apple support for additional information on changing, adding, or removing Apple ID payment methods.