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.
If you get error message 20008 ("purchasing not available on this device"), most likely you have an active Yousician subscription linked to your Apple ID. Go to Settings → Account in-app and select the Restore button. If that doesn't work, reach out to our support team.