iPod classic permissions: This is a new problem.

iPod classic permissions: This is a new problem.

I get this warning when syncing my iPod to my iMac "(Name of Song) was not copied to this iPod (Name of iPod) because you are not authorized to use them on this Mac.

Sometimes I get:

You must authorize this Mac for the account used to purchase this item.

Does anyone know how I can authorize my Mac to play the music that I own?

Thank you.