The script in the Unity Editor will try to run:
keytool -exportcert -alias androiddebugkey -keystore %HOMEPATH%\.android\debug.keystore | openssl sha1 -binary | openssl base64
(or the mac equivalent if you are on a mac)
see: https://developers.facebook.com/docs/android/getting-started/facebook-sdk-for-android/
from your command it looks like you should have the key in:
"C:\Users\(My UserName)\.android"
and not
"C:\Users(My UserName).android"
(notice the folders)
or if you have multiple hard drives and don't have your home on your primary drive, then move debug.android to where ever your home variable path points to.
another thing would be to ensure that both keytool and openssl is in your path. you can manually execute them in the shell. If it can't find them then add them to your PATH.
I hope that helps.