AppInstaller protocol not recognised by browser

jonrmorganI'd relied on this info. in the docs, and inferred the app installer would already be on my system:

It wasn't so I just downloaded it from the Store.

Next up then - the installer runs but can't open the package:

I can find very little troubleshooting information for this error and don't know where to start debugging. Here's my installer file:

<?xml version="1.0" encoding="utf-8"?>
<AppInstaller
Uri="https://localhost:5001/downloads/XYM_MSIX_x86.appinstaller"
Version="1.0.0.0" xmlns="http://schemas.microsoft.com/appx/appinstaller/2017/2">
<MainPackage
Name="82601cfc-7b41-4868-848f-b52ad84e9789"
Version="1.0.0.0"
Publisher="CN=Dadso"
Uri="https://localhost:5001/downloads/XYM_MSIX_1.0.0.0_x86_Debug_Test/XYM_MSIX_1.0.0.0_x86_Debug.msix"
ProcessorArchitecture="x86" />
<UpdateSettings>
<OnLaunch
HoursBetweenUpdateChecks="0" />
</UpdateSettings>
</AppInstaller>

Any help to get going on this much appreciated !