This computer is already associated with an Apple ID

Have you recently used the computer to download past purchases from a different Apple ID? I suspect the computer is associated with one ID and you're trying to do something that wants to associate it with another.

Authorization vs. association

Up to 5 computers can be simultaneously authorized for each Apple ID. There is no direct record of which machines are authorized. It is up to you to keep track. iPods & iOS devices are limited to DRM protected content from up to 5 different accounts. There is no limit to the number of devices that you can load up with content using iTunes. You can deauthorize computers that you no longer wish to use to access protected content so that you can authorize others. You either do this on the specific machine or deauthorize all and then reauthorize as needed. Having used deathorize all you cannot use it again for a year, though Apple Support may be able to perform a reset if you can explain why you need to use the feature again before it becomes available to you.

HT201251: Authorize your computer in iTunes - Apple Support

A mix of up to 10 computers and iOS devices (no more than 5 computers) can be associated with your Apple ID for iTunes Match/Apple Music or automatic downloads of past purchases. There is a 90 day limit before a computer/device can be switched to another ID or the association removed. The list of associated devices is the one you can see within your account details in iTunes. You associate a device or computer by enabling certain features, typically automatic downloads of past purchases. You don't necessarily need to associate a device with your Apple ID to use it with your content.

HT204074: View and remove associated devices in iTunes - Apple Support

You can happily remove any and all associations that are not within the 90 day limit. Those that you actually use will be recreated as when you make use of a service that triggers association.

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