-45054 errors occur when permissions on files or folders that iTunes uses to download purchases are restricted. The full -45054 alert message reads: “We could not complete your iTunes Store request. An unknown error has occurred (-45054).”

To resolve this issue, follow the steps below:

1. If it's open, quit iTunes.
2. Open Finder.
3. From the menu bar at the top of your computer screen, choose View > As Columns.
4. From the same menu bar, choose Go > Go to Folder.
5. In the window that appears, type the phrase:
   /Users/Shared/adi
6. Click Go.
7. Drag the adi folder to the Trash.
8. Open iTunes and try to make a purchase.