Help No account is currently storing backed up data

[pjstock]

This question has been asked a thousand times, but I don't see an intelligent reply or solution yet, so let me try.
My Lenovo Yoga YT3 tablet running Android 5.1.1 is prompting me for a System Update "you system can update to the latest version").
But, it is also warning me, scarily, that "this update will erase ALL data.... please back up etc.etc."

However, under Backup and Reset I am getting the message above:
even though I have 2 accounts setup under Google.

How do I associate one of these accounts (ideally a different one than my phone is backing up to) for backup storage?

thanks
Peter