Hold on... don't delete this yet! I have the same problem and questions about my MS Store msfs2020 installation.

So, first thing I run into doing the install of the 777, the installer wants to know where MSFS2020 is installed. Hummm... I'd never been asked befre for that information during an install, or for that matter, for any reason. I had no idea where it was installed, but after a little Google research I found that if I had purchased from the MS Store, by default, it is installed in C:\Users\******\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\Packages\Official\OneStore. That's what I entered and the install finished with no issues. After booting up the Sim the 777 loaded and ran fine. (At least as fine as it is designed to run... but that's a whole other post).

Then I tried to run the ACE app. A window popped up informing me that "Path to your 'captainsim-aircraft-m773' folder is OK.". Great, so my information was correct for the MSFS2020 install path. Then right below that line it states in red...
"Cannot find a path to your MSFS2020 OFFICIAL folder. Please select it manually"

it goes on to say...
For Microsoft it's /OFFICIAL/ONESTORE/
For Steam it's /OFFICIAL/STEAM/

That's odd, I think to myself. It knew the path to the OFFICIAL folder when it did the installation. How could it have forgotten so fast? Anyway, I entered /OFFICIAL/ONESTORE/ exactly like the hint showed, but it wouldn't let me save that. Well, I thought... they must want the entire path on the C: drive to the OFFICIAL folder, so I entered C:\Users\*****\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\Packages\ which is the path to the OFFICIAL folder. Nope... wouldn't save that either. I added the folders to the path then C:\Users\*****\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\Packages\Official\OneStore but still won't let me save it.

So, what is the correct formula? This is the most confusing and poorly designed install method  I've ever seen.