[Zubersoft]

For anyone else that checks this forum post, in order to use YubiKey (and similar products), you have to turn on Advanced Protection for your Google account. This limits third party access to things like Google Drive. MobileSheets uses a sensitive scope in order to be able to access all Google Drive files and folders, so it is being denied access when Advanced Protection is turned on. As far as I can see, there is no way around this. You would have to use Import->System File Browser and go through the Google Drive app if you need to have "Advanced Protection" enabled.

Mike