This error occurs when you are trying to add some comment or something to the Pull Request which has been already created and this can be easily fixed by adding appropriate permissions to the user account which is trying to do that. The user account in question should be part of a group which has “Pull Request Contribute” permission.
TF401027: You need the Git ‘PullRequestContribute’ permission to perform this action
To fix this error, Click on Project Settings –> Repositories –> Security and check whether the Project Collection Build Service has Pull Request Contribute permissions , if the group doesn’t have the permission then the set allow the permission for Project Collection as shown in the below screenshot.
