Important Note from Microsoft;
We continue to upgrade our Cloud offerings, including the Project Online Desktop, to provide our customers with the latest capabilities and up-to-date online experiences. Due to these ongoing updates, beginning September 15, 2018, we will be removing the ability for the Project Online desktop client to connect to those versions of Project Server outside of mainstream support, including Project Server 2013.
Overall, this milestone is part of the end-of-life journey for Project 2013 Standard, Professional and Server. Prior to this, support for the connection between Project Professional 2013 and Project Online ended on August 31, 2017 while April 10, 2018 marked the end of mainstream support for Project Server 2013.