I've had my macbook air since the end of summer 2014 and have never once been able to purchase music or anything from itunes.
On all other devices it works, I'm logged in with my apple ID and the computer is authorized.
It's always the same, I can click to buy something and it will have me input my password but then as it's processing the request the error message always pops up and it's driving me insane.
MacBook Air, iOS 8.3
iTunes error -45054 - https://discussions.apple.com/thread/5469367?answerId=23478840022#23478840022 - permissions error, try repairing disk permissions using Disk Utility.
Using Disk Utility to verify or repair disks - http://support.apple.com/kb/HT1782 - Ignore false warnings this may produce.
If repairing permissions using Disk Utility does not work, try: iTunes: Missing folder or incorrect permissions may prevent authorization - http://support.apple.com/kb/TS1277
This doesn't work for everyone, it didn't fix the error for me. Updating to the latest version of iTunes fixed the problem for me.
Can't purchase music on iTunes error -45054
