This error would occur during creation of pull request to merge one branch with another branch. This error occurs when there is a repository that has same name in another project (or) folk of an existing repository in another project. Basically if there is a repository called “A” in Project “AAA”, and if there is a fork of repository called “BA” in another project called “BBB”, this error would occur.
The error occurs as both the repositories are the same and Azure DevOps doesn’t know which repository has the pull request. The only solution for this is to rename any of the existing repositories to another name. Once any of the repositories has renamed, this error would just disappear.