I can’t login my MacBook air

I think it may be because the string that stores your password may be corrupted. What I suggest you do is you shut off your computer, then press and hold the Touch ID/power button until it says 'Loading Startup Options'. After that, there will be a choice between "Macintosh HD" and "Options". Click on options, then login, if it still says "Incorrect Password", click on "Forgot All Passwords", then follow the instructions. :)