This issue occurs when there is currently a Core+ upgrade undergoing on your system, so the Upgrade Mode setting is currently on.
If you are expecting an upgrade on your system, your system will be set in Upgrade Mode, so this message will appear in a bar at the top of the page.
However, if your upgrade has finished, as this has been confirmed by the Upgrade team, you can then disable Upgrade Mode by logging in as the role assigned to log in during an upgrade and selecting the Turn Off Upgrade Mode option found at the top of the screen in the orange bar.