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/*