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