As this is the first thing that comes up on Google when searching this error, I wanted to add the answer I found that worked when I stumbled upon this issue.
In short: I didnt have write access to the repo.
This could be the same reason as from this post: https://answers.atlassian.com/questions/125892/submodule-with-different-user-fails-to-clone-push-with-sourcetree
In short, it might be because you're using HTTPS authentication and not specifying your username in the URL. Have a read and see if it helps.
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