You can use --allow-unrelated-histories to force the merge to happen.

The reason behind this is that default behavior has changed since Git 2.9:

"git merge" used to allow merging two branches that have no common base by default, which led to a brand new history of an existing project created and then get pulled by an unsuspecting maintainer, which allowed an unnecessary parallel history merged into the existing project. The command has been taught not to allow this by default, with an escape hatch --allow-unrelated-histories option to be used in a rare event that merges histories of two projects that started their lives independently.

See the Git release changelog for more information.

More information can be found in this answer.

I ended in the same situation after a series of mishapps. Have a look at this thread : I just did a 

git pull --allow-unrelated-histories

and it was fixed!

this happens when your local repo and the remote repo have different root commits (separate histories). if you are sure you want to combine them, run:
git pull origin main --allow-unrelated-histories
Resolve any conflicts, commit the merge, then push:
git push origin main
if you intended your local repo to exactly match the remote (and you don’t need local commits), the safer approach is to discard the local history and re-clone the remote repository instead.