In fact, there seems to be two potentials problems. Credentials for private repo referenced as SSH (git@github.com:[…]/…) is one and others pointed at the solution in this thread.
But it turns out you may have package referencing others as HTTPS (https://github.com/[..]/…), I ran into this issue and got lucky finding this topic . It turns out it works super fine to just for Xcode to find its credentials to use Authenticate host with netrc.
Hope it helps.