Q: I had bought the garage band lesson from sting. I have a new mac now. Where can I get the lesson again?

Try to restore the lessons from your old Mac.

Copy the folder /Library/Application Support/GarageBand/Learn to Play/Artist Lessons/  from the old mac to the corresponding folder on your new Mac.

Then restart your Mac and check, if you now can access the artist lessons.

If you cannot restore the Artist Lessons from your old mac or a backup of your old mac, try to download again from the Apple Store. See this Support document:   Re-download GarageBand Learn to Play Artist Lessons - Apple Support