This error message suggests that there is a problem with the SSH key used to authenticate with the remote repository. Make sure that you have generated a new SSH key pair and added the public key to your GitHub account. In your Forge server, check that the correct private key is added to the ssh-agent by running the command: ssh-add -l. If the correct key is not listed, add it by running ssh-add ~/.ssh/id_rsa
Test your connection and if you still can't deploy, try adding the GitHub host key to the server's known_hosts file:
ssh-keyscan github.com >> ~/.ssh/known_hosts