Solution: Home Sharing could not be activated because an error occurred (5599)

I have been setting up a brand new iMac and when I tried to turn on Home Sharing in iTunes (File>Home Sharing>Turn Home Sharing On). I got the error message:

"Home Sharing could not be activated because an error occurred (5599)"

I had already successfully signed into my apple account in iTunes (Account>Sign in) and to the App Store. There is another Mac on the same network, with the same Apple ID, that works fine. I then tried to:

Turn off Home Sharing on the other Mac

Restart iTunes

Restart the iMac

I have seen other reports of this problem but could not find a solution. Finally, I discovered that I had not authorised the computer for iTunes. Once I did this I was able to turn on home sharing.

Hope this helps somebody.