User is trying to add a phase to one of the projects, but gets an error that the Phase ID is already in use.
This happens if an existing project or phase already has the ID that you are trying to set. In Core, phases are also projects and the IDs of all the projects in Core should be unique. In other words, Phase IDs have to be unique not only across the related phases, but across all projects. So it does not allow you to set the same phase ID if you have used it before on another project.