The main reason for this kind of error is your previous pull failed to merge automatically and went to conflict state. And the conflict was not resolved before the next pull
To remove this you need to undo a merge and pull again
git merge --abort <since Git version 1.7.4>
git reset --merge<for Git version less than 1.7.4>
And then resolve the conflict and don’t forget to add and commit the merge then after that git pull command will work fine.