When using a private repository in a Go project, you need to provide authentication. This is straightforward and documented](https://go.dev/doc/faq#git_https).
On Mac, other than using an netrc file you can use the OSX toolchain.
git config --global credential.helper osxkeychain
Moreover, and this was not clearly documented for me, you also need to tell go get that it is working with a private module, so that it doesn’t try to check against the public checksum database at sum.golang.org:
go env -w GOPRIVATE={private_repo}
Source: https://play-with-go.dev/working-with-private-modules_go119_en/