A common cause is cloning using the default (HTTPS) instead of SSH.
You can correct this by going to your repository, clicking
"Clone or download", then clicking the "Use SSH" button above the URL
field and updating the URL of your origin remote like this:
git remote set-url origin git@github.com:username/repo.git
You can check if you have added the remote as HTTPS or SSH using:
git remote -v