I am not the OP, but have exactly the same error. When I copy/paste the command into the command window it works. It asks me for user name and password and I use the exact same ones I have set up in SourceTree 3.4.2. I have removed and re-added the credentials several times. Usually, I would just type it both places but to absolutely ensure it isn't a typo I have gone through the sequence a couple of times using Google password manager as the source to copy/paste the password. Absolutely using the same username/password combo.
EDIT - It is SSH in both cases, but I have also tried HTTPS