Authorize a Mac to play iTunes Store purchases
To play many purchases from the iTunes Store, you must authorize your computer using your Apple ID and password. (Authorization helps protect the copyrights of the purchased items.)
You can authorize or deauthorize a computer at any time.
Authorize a computer to play iTunes Store purchases
You can authorize up to five computers (Mac, PC, or both) at a time. To play a purchased item on a sixth computer, you need to deauthorize another one. An iPhone, iPad, iPod, or Apple TV doesn’t count as a computer.
In the Music app on your Mac, choose Account > Authorizations > Authorize This Computer.
If requested, enter your Apple ID password to confirm.
Deauthorize a single computer
Be sure to deauthorize a computer before you sell it or give it away. This prevents it from continuing to count as one of your authorized computers and from playing your iTunes Store purchases.
In the Music app on your Mac, choose Account > Authorizations.
Choose Deauthorize This Computer.
Deauthorize all authorized computers
If you forgot to deauthorize a computer you no longer own, and you have five authorized computers, you can deauthorize all your authorized computers at once. You can use this method once a year.
In the Music app on your Mac, choose Account > View My Account, then sign in (if necessary).
On the right, click Deauthorize All.
If you have trouble deauthorizing a computer, see the Apple Support article Authorize or deauthorize your computer for iTunes Store purchases.