This error shows up when two different projects are merged (i.e. they both could be the same project but unaware of each other's existence and have different commit histories).

If you are facing this, chances are that:

1. You must have cloned a project and the .git directory must have corrupted or got deleted and at this point, Git is unaware of the changes being made and will throw this error when you try to push to or pull from the remote repository.

2. You created a new repository, made some changes and added the commits and then you tried to pull from the remote repository.

Well, you can easily resolve by passing the --allow-unrelated-histories flag when pulling the latest updates from the remote repository:

git pull origin master --allow-unrelated-histories

Hope you found this useful!