As explained by MacRumors, here is the flow:

• Click on System Preferences.
• Click on App Store.
• Click on the padlock icon to lock it if necessary.
• Click on the padlock icon again.
• Enter your username and any password.
• Click Unlock.

The stakes on this page are low so it's not the end of the world, but COME ON APPLE. If I was in their QA team I would make a special task to look at every single place in macOS where authentication happens and regression test the hell out of it.

I'm not particularly interested in whether Apple's software is better or worse than before, but there are a a ridiculous number of authentication issues in Macs revealed in just the last few months and it's hurting Apple's reputation. One of the selling points of a Mac is that you need to do less to secure it than a Windows PC, but lately there have been multiple ways someone could just walk up to my computer and just blow through password prompts.

This one event isn't the end of the world, but this is how reputations degrade over time. Apple needs a software win soon, because it's really just been a streak of bad news for them for months.