Help "Change" button is unavailable in App Storage settings to move apps to SD Card

[shibulal]

I'm using Moto G3 with marshmallow 6.0.1. Recently I mounted a 32 GB SD card and using it as a Portable device in my mobile. When I tried some of my downloaded apps to move to my SD card, I went to Settings ->Apps. There I chosen an app, then tapped on Storage option, there I don't see "Storage Used" option and the "Change" button through which I can opt for SD Card. I checked for all downloaded apps and none of the apps having these options. Can anyone help me to see why I'm not getting an option to change the storage location for any of the installed app?..
Thanks in advance!