FYI, if you’re trying to use android adb (or some tool that uses adb, like helium backup), once you trigger the backup process from the computer, it should open a dialog on your phone. The dialog will ask you to confirm the backup process, and asks for your backup password, and a password to use to encrypt the backups (they can be the same password). Then there are two buttons at the bottom, notably “Back up my data”, which is the one to press to continue.
The most-recent time I tried to do this, I was unable to press the “Back up my data” button. Someone online pointed out that screen-overlay apps (like the Twilight app that makes your screen dimmer and more reddish after sunset [to help with sleep]) will interfere with this button. After pausing my Twilight app, it started working.
It’d be really nice if there was some sort of notification that this particular button is special and is unclickable if you have a screen overlap app :-p