When you click Publish, a dialog box prompts you to save your project plan.
You have made changes to your local project plan but you have not saved the changes to Project Server.
If you make changes to your project plan, you cannot publish the updated project plan without first saving the changes. If you do not want to keep your changes, close the project plan without saving. Then, open the project again and publish it to Project Server. Otherwise, save the project and then publish.