Following steps must be done for FlexDeploy Server and Endpoint where Git commands will be executed.

Locate or generate default SSH keys.
Add SSH public key to GitHub account. You must use id_rsa.pub key for this purpose.
If GitHub is SSO enabled
1. Click on enable single sign-on
2. Click Authorize
3. When routed to SSO provider, enter SSO credentials
Test SSH connection using ssh, and perform host authorization.
Configure FlexDeploy
Click Test Connection to verify configuration