You're so close to starting your lessons - no way this is going to be what stops you! Apple manages all these payments, so there are just a few quick things to check on your Apple ID.
Your purchase failed
Check to see if you have in-app purchases enabled:
Go to device Settings
Tap Screen Time
Tap Content & Privacy Restrictions
Turn on Content & Privacy Restrictions
Tap iTunes & App Store Purchases
Tap In-app Purchases
If your purchases are enabled, try to:
Log out of your Apple ID
Log back in again with your Apple ID
Restart your Simply Guitar app
Try purchasing again
Your purchase was declined
Have more questions? You can talk to us through the app. Tap on Menu > Have a Question?