Seeing the same thing. I've created a key in the NodeRed general settings, I see the keypair in ~/.node-red/projects/.sshkeys folder and have added the public key to my Github keys in Account Settings. I attempt to Manage Remote branch that I've added in the Project Settings as ssh://firstname.lastname@example.org:ikidd/"myreponame".git and it says authentication required and just keeps showing my SSH Key in the dropdown but never logs in. If I cancel, it doesn't let me choose a branch or create an origin:main link.
I've added the github public key for their SSH host to known_hosts in the service users .ssh folder by attempting an ssh login to email@example.com and saying Yes when it verifies the identity of the server.
I've changed the remote repo between private and public and restarted nodered, does not seem to help.
I have had this work, but had to do some manual git linking at the projects local repo folder and still had to come into the NR page and link them, but at least at that point it prompted me to Create origin:main link to click on.
I'd prefer to figure out whatever method the Project developers were thinking of to get these repos to link and enable pushes.