I decided that the famous GitHub error Permission denied (publickey) is worth a blog entry, even if a short one.
We all heard the useless tipp: 'Go and add your ssh-keys in the GitHub FrontEnd.'Let me suggest a different solution entirely. It might very well be that you mistyped your project name when adding it to your local git.
Remove and Add Origins
Let's remove the old name:
git remote rm origin
Add it again, and don't mistype this time:
git remote add origin firstname.lastname@example.org:Username/Projectname
Voilà I hope that helps.