As suggested by the title, the Project Online client requirements are changing! The reason being that Microsoft are making architectural changes to features in Project Online which impact older versions of Project that currently connect to Project Online.
You can see the original notification from the Office 365 Admin Portal below:
To summarise, as of February 28, 2017 Project Professional 2013 and Project (Pro) for Office 365 (2013) will no longer be able to connect to the Project Online service in Office 365, you will need to be using a current version of the Project Online Desktop Client (Project Professional 2016 or Project for Office 365).
There are other licensing considerations too, and the table below highlights which versions of Project 2016 can be installed on the same computer with Click-to-Run (Office 365) and MSI (on-premise) versions of Office 2016 and Office 2013:
Further information on supported scenarios is available at:
If you have licensing questions, we would recommend that you speak with your licensing solution provider (LSP).