Failed to add extension to virtual machine

Hi all,

I am trying to add the Antimalware extension to a virtual machine for protection but when i try to add the extension i get the following error:

Failed to add extension to virtual machine. The virtual machine request is invalid The specified source image is a user image. The image must be a platform image.

I have installed the VMAgent.

Can anyone recommend a way to install the extension to this virtual machine?

Thanks!