When you have two unrelated projects (two different repositories), and you try to pull and merge files, The “fatal: refusing to merge unrelated histories” Git error occurs.
Solution: use –allow-unrelated-histories
git pull origin branchName --allow-unrelated-histories
Sources:
https://www.educative.io/edpresso/the-fatal-refusing-to-merge-unrelated-histories-git-error
https://stackoverflow.com/questions/37937984/git-refusing-to-merge-unrelated-histories-on-rebase