.appinstaller error - schema is not valid for OS

Question

I'm getting the following error when attempting to install my .msix file via an .appinstaller file.

This .appinstaller file is using a new schema which is not valid on this OS version. Please update to the latest version of Windows 10.

• I've tried various schemas /2017, /2017/2, /2018, 2021.
• I've tried with 'Developer Mode' switched on & off.

There doesn't seem to be anything relating to the error in the event viewer in AppxDeploymnet-Server.

This is my .appinstaller file.

<?xml version="1.0" encoding="utf-8"?>
<AppInstaller
    xmlns="http://schemas.microsoft.com/appx/appinstaller/2017/2"
    Version="1.0.0.0"
    Uri="https://mywebsite.com/myapp.appinstaller" >
  <MainPackage
      Name="myapp"
      Publisher="CN=myOrganisation"
      Version="1.0.0.0"
      ProcessorArchitecture="x64"
      Uri="https://mywebsite.com/myapp.msix" />
  <UpdateSettings>
    <OnLaunch
        HoursBetweenUpdateChecks="0"
        UpdateBlocksActivation="true"
        ShowPrompt="true" />
    <AutomaticBackgroundTask />
    <ForceUpdateFromAnyVersion>true</ForceUpdateFromAnyVersion>
  </UpdateSettings>
</AppInstaller>

This is the version of Windows I'm using.