Make one like this, creating an e.g. "modules" directory to clone files inside a modules directory from another repo:
git subtree add --prefix modules git@bitbucket.org:something/something.git main --squash
If it's still complaining about the directory not being a repository, e.g. this error:
fatal: 'modules' does not appear to be a git repository fatal: Could not read from remote repository. Please make sure you have the correct access rights
...make sure this is in .git/config:
[remote "modules"] url = git@bitbucket.org:something/something.git fetch = +refs/heads/*:refs/remotes/modules/*