This is quite a common error which comes when you are trying to merge two branches without any common base.
Initially, "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.
So you need to add
to remove this error. So suppose you are doing a git pull from origin :
git pull origin name_of_branch --allow-unrelated-histories