I've tried to reinstall an artist lesson from my order history.
Occasionally this has worked, but right now it does not work - it may depend on the country you are in.
Do you have a backup of your system from before the reinstall?
The artist lessons are installed in the system library. If you have a backup, restore the folder
/Library/Application Support/GarageBand/Learn to Play/Artist Lessons/
This way I could restore my "Roxanne" lesson by Sting recently