I purchased an album and six of the fifteen tracks will not play. I get a message that this computer is not authorized, while all other tracks play fine.

Repeated requests to authorize an already authorized computer for store downloads typically indicates you have corrupt or incomplete copies. Backup and then delete the tracks that won't play from your library without hiding from your purchase history. Redownload using the cloud links that should remain, or close iTunes and reopen if they don't show up straight away. While downloading click the activity widget and turn off the option that allows for simultaneous downloads.

If that hasn't helped see If iTunes for Windows prompts you to authorize your computer when you try to play purchases - Apple Support.

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Try this Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

good luck!