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If you're running macOS High Sierra, try this:[macworld.com...]
Open System Preferences. Click on App Store. If the padlock is unlocked, click to lock it. Click the padlock to unlock it. In the prompt, enter your username and any password.
The App Store preferences pane should unlock. We tried it on a new iMac and MacBook Pro, both with macOS 10.13.2, and it worked.