I'm an ordinary bitbucket user who plays with git push git pull and git merge commands and never come across this weird situation.
Now I'm trying to merge my feature branch into my master branch and getting this error:
From bitbucket site follow these steps:
Go to Compare
Select Destination and Source branches
Select Compare button
Select Merge button on top right corner
This error was a result of a bug that has now been fixed.
来源：https://stackoverflow.com/questions/42533093/bitbucket-unable-to-merge-this-field-is-required