Why do I have to sign the password every time I tried to log in?

There re various passwords around.

Where are you signing in with your password?

Into the iPhone with the initial passcode? If so, set up Touch ID on your iPhone 7.

If you’re specifying a different password elsewhere, which password prompt?

Are you being asked to update the Apple ID Settings? If so, some people have fixed this by signing out of iCloud, restarting their device, and then logging back in. That hasn't worked for everyone.

There have been posts that stated this was a problem that Apple is aware of and is trying to fix.

Settings - Update Apple ID Settings When It Won’t Go Away.    

Settings - Update Apple ID Settings When It Won’t Go Away (2).