Disclaimer: I am a consultant at Amazon Web Services, and this is my personal blog. The opinions expressed here are solely mine and do not reflect the views of Amazon Web Services (AWS). Any statements made should not be considered official endorsements or statements by AWS.
Normally, you get this error message when you are performing GIT operations from two different places. Like I got this issue while I was using Git Bash (Command Line) & Visual Studio together to perform Git operations.
merge session is going on in one of the two instances of GIT. i.e. Git Bash & Visual Studio Git.
merge session in one of the two opened instances.
Note: When I got this error message, I found that a window with commit message text-box was opened in Visual Studio. I just entered the commit message & committed the code. Later I also closed Git Bash window as well.