Noticieboard events have always been tricky to schedule.
I encountered a similar message several months ago when planning a drop party for achieving T4. I kept moving the time out further until it would stick.
Make sure you have at least 25 hours between the time you're posting and your event, and make sure you're on year 2018. I remember once the year kept sliding back on me after I selected it, but I've not scheduled any events yet this year.
If you still can't get it working, be sure to file a bug report to get it looked at.
22-Feb-2018 13:12:56