You need to make sure the you are running MS Store version: 11906.1001.24.0
in order to be able switch out from S Mode
Sometimes even though after update the MS store to the latest version you could face this other issue:
In that case you will need to download the ISO file and run the Setup from Sources Folder [That will be like Custom Install].
When this process finish The Get option will not be longer grey out and you can switch out from S Mode.