Can't download updates because "This update is not available for this Apple ID"

I keep getting this message when I type in my password to do an update.

This update is not available for this Apple ID either because it was bought by a different user or the item was refunded or cancelled.

The computer was bought by me and has only been used by me. What do I need to do to fix this?

Try going to the "purchases" section in the App Store. Accept the free apps like iMovie and Garageband there.

If you logged in to a new Mac with an existing Apple ID go to the App Store as Non Plus Ultra already answered. You should accept the free apps that came with the new Mac and link the license to your Apple ID.

In the new App Store you should go to your account, which can be found at the bottom of the menu on the left in the App Store. You will see all your previous purchases and a notification box including an Accept button. Click the button to add the license to your Apple ID.