Hi Rob. My name is Lester, and I am an Independent Advisor. I understand the difficulties you've been facing, and I want to assure you that I'll be giving my all to provide the help you need.

When did the issue start to occur? To run the Windows Store Apps troubleshooter, alternatively, you can also type 'Fix Windows Store' in your search bar next to the start menu and then press Enter. Follow the prompts to run the troubleshooter. Thank you.

I hope this information is helpful. Please don't hesitate to reach out if you have any more questions.

After visiting innumerous resources, this one (which I hope google will index it first soon, given it's the only one that works (see comments there)) finally solves it.

In my case, the steps were not exactly the same, so here is a recap of what worked for me, but based on the solution above:

1. I did not have XBox app installed, so googled for it and eventually downloaded from here.
2. The original solution mentions a Settings tab, which I do not see, I guess the app was upgraded since then. Anyway, the same can be achieved by following the recommendation pop-up at the bottom - clicking at Fix them in settings (see screenshot below)

3. Then the following window appears. Choose to install App Store (mine is missing because I am screenshooting AFTER solving the issue). As opposed to the original solution, I only installed this one, and it suffices.

That's it. You should have App Store now.