iTunes requesting password to create the very first iPhone backup This question is similar to others e.g. here but I still can't find a solution.

I am trying to back up my iPhone for the very first time on a brand new MacBook Pro.

When I log in to iTunes to initiate the backup I see the following message noting that there has been no backup before:



Then, if I try to turn off encryption of the backup (or indeed set a password), I see:



Does anyone know what's going on, or what I can do to back up my iPhone and then restore to a new one?
The encrypted backup password is stored on the device itself, not in iTunes.  This indicates that you have used a different iTunes previously to make an encrypted backup. 

If you don’t know the encrypted backup password that is stored on the device, the only option is to enter Settings on the phone and ‘Reset all Settings’ which will preserve user data, but restore all settings to default.

See this Apple support article.  (Password reset info in the last section.)