If you have a work GitHub account in addition to a personal one, you most probably need to clone, pull from, and push to from repositories with multiple different SSH keys. This post talks about how to manage them.

The main issue is that GitHub does not allow the same SSH key to be used in multiple user accounts. So if the SSH key you use in your personal account is at ~/.ssh/id_rsa, then generate a new one for your work using ssh-keygen. In that process, when it asks you the path where that key should be created, give ~/.ssh/work_key. Add this key to your work GitHub account.

Put all your work repositories in one directory, for example ~/work. Next, add the following lines to ~/.gitconfig:

[includeIf "gitdir:~/work/"]
  path = ~/.gitconfig_work

Note the / at the end of the directory path, which includes all subdirectories. Finally, create ~/.gitconfig_work with the following lines:

[user]
  email = <YOUR WORK EMAIL ADDRESS>
  name = <YOUR WORK NAME>
[core]
  sshCommand = ssh -i ~/.ssh/work_key

The config makes git SSH with the key in ~/.ssh/work_key when cloning, pulling from, and pushing to repositories whose .git directories are under ~/work.

(page source)