If you are using Visual Studio Online and setup your project using TFS doing a code review is straightforward in visual studio.
However ff you are using Visual Studio Online and setup your project Git you might wonder how you can conduct a code review. You may do a quick search and then try ‘new pull request’ . You might get a weird error message you don’t understand.
TF401189: The source branch has been modified since the last merge attempt
Then you do another search (bing or google) you won’t find anything useful.
After trial and error I found that the root cause is that I didn’t use a topic branch. It is that simple.