I dont know whether your Office version is genuine or not (I hope it is, where did you get it from?) but I suspect that message is NOT genuine! I've never seen a non-genuine message that says that "in 15 days, Office applications will be marked as not genuine" or "genuine Office helps guard against vulnerabilities". That looks a highly made up message to me.
I suggest you run a malware scan with free Malwarebytes Antimalware and let it clean any infection it finds. Then go to the Microsoft genuine website and validate your copy of Office. If there is any issue, take it up with MS or your vendor. DO NOT click on the buttons in that popup message, they may be highways to trojans.