Unfortunately, by default
MSIX is not supported on Windows 7.
You can see the support OSs matrix of MSIX here:
MSIX support on Windows 10 version 1709 and 1803
The MSIX support was officially released with the Windows 10, 1809 update. Support for previous Windows (10) versions was added later on, requiring the latest version of the AppInstaller app from Microsoft.
You can find more information about this in the "MSIX Supported Features by OS Version"
, due to a number of inquires on this, Microsoft has released MSIX Core
. Basically, MSIX Core brings MSIX support to version of Windows earlier than Windows 10, version 1709.
Hope this helps.