First check at cPanel SSH Access there should not be any key if there is any delete this than generate new key from terminal ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa
After generating key Go to .ssh directory and open .pub file and copy SSH Key Add this SSH Key into GITHUB REPO - Setting - Deploy Keys
Backup all files from public_html to _backup directory e.g. /home/_backup