WiX Bundle creation to install .NET Client for .NET5.0 application

I have been struggling to build a WiX bundle to handle shipping the .NET 4.6.1055 client installer EXE file. I can get the .msi to be built and I can see the NDP461-KB3102438-Web.exe file in the installer folder... but when I try to run the .msi I am told that it could not be opened and that I should contact the application vendor (me?) to verify that the installer package is valid.

The .wxs file is this:

<?xml version="1.0" encoding="UTF-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
    <Bundle Version="1.0.0.0" Manufacturer="..."
       UpgradeCode="...">

        <BootstrapperApplicationRef Id="WixStandardBootstrapperApplication.RtfLicense" />

        <Chain>
            <ExePackage Id="Netfx45Xxx" Cache="no" Compressed="no" PerMachine="yes" Permanent="yes" Vital="yes" InstallCommand="/q"
                SourceFile="C:\Workspace\Fusion Wireless System Path Estimator\.NET5.0.3\NDP461-KB3102438-Web.exe"
                DetectCondition="(Netfx4FullVersion=&quot;4.6.1055&quot;) AND (NOT VersionNT64 OR (Netfx4x64FullVersion=&quot;4.6.1055&quot;))"
                InstallCondition="(VersionNT >= v6.0 OR VersionNT64 >= v6.0) AND (NOT (Netfx4FullVersion=&quot;4.6.1055&quot; OR Netfx4x64FullVersion=&quot;4.6.1055&quot;))"/>
        </Chain>
    </Bundle>
</Wix>

The project has the references added to WixBalExtension, WixNetFxExtension and WixUIExtension. What have I done wrong or left out, or put in that shouldn't be there? Note that this project is STANDALONE and not part of another installer project (yet, that will come next once this one works!). The file I'm including is a .EXE file not a .MSI package