Scheduling new posts error

Kham1989​ and MyUsernameNotAvailable​ and Helen Cummings​ 

If your issue is that you're using a custom content type in the SitePages library, the following should get Scheduling working for the custom content type.

This was my reference for the following:
https://learn.microsoft.com/en-us/sharepoint/troubleshoot/sites/cant-schedule-publish

in contenttype hub > site pages > settings
turn on versioning, major and minor versions

in /sites/contentTypeHub/SitePages/Forms/AllPages.aspx
turn on Scheduling

in SharePoint admin centre
https://xxxx-admin.sharepoint.com/_layouts/15/online/AdminHome.aspx#/contentTypes
find the content type "Comm site page" in my case

When you open it you'll see "Publish Start Date" as a new column ( I assume this is because my custom content type is based on the default "Site Page" content type, so it has inherited this.)

Publish > republish

should take 15mins to flow down to all the libraries using that content type

reference for all this (except the turn on major minor versions)
https://learn.microsoft.com/en-us/sharepoint/troubleshoot/sites/cant-schedule-publish

Kham1989​ and MyUsernameNotAvailable​ and Helen Cummings​ 

I've just had one of my editors tell me about the same problem.
Create a News Post from a template, add some content, save draft, then edit again, click Page details and then the "Scheduling" drop down is blank.

In our case the News Post template, was a specific content type i.e. not the default "Site Page" content type.

After changing a file that had a blank "Scheduling" drop down, back to the default "Site Page" content type the "Scheduling" drop down was no longer blank and could be set.
I'll come back with a reply if I can find a way to enable Scheduling with a custom site pages content type.

Helen Cummings I think you first have to go to : Left Navigation - Site Content - site pages library . There you can find the several pages (every news post IS a page). 

Pls. click in the top menu bar of that Pages library on that button "Schedule". Then there is a window and are asked to activate the scheduling for this library. By clicking OK, each of your pages has also to be activated on that scheduling. 

So: pls. select one of your pages - click again on that upper button "Schedule". In the Details panel on the right you can activate now the scheduling for this selected page, activate a certain date and time. And there you are! So there are 2 steps to activate the scheduling: step 1 is the activation in the whole lbrary, step 2 is selecting a page and activate it by clicking once again the schedule button and defining the date &time details in that page details. Hope that helps! greets from Eva.