Error in iPhone 6s "iPhone is disabled. Connect to iTunes"

Here is what to do: If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

It’s also possible this iPhone is simply failing.

Note that if you are using a Mac running Mojave or earlier, you would use the iTunes application – rather than the Finder – to help reset the phone. The Apple Support article no longer mentions the Macintosh version of iTunes, but it was the original application for managing iPhones.