I had an issue where I could no longer create site pages in my site pages library, the option wasn't there although the content type was in the library.
The cause was an admin setting "Allow users to create site pages" which we had turned off when reviewing what users should be allowed to do in Office 365 and SharePoint.
- Go to Admin
- SharePoint admin
- Ensure "Allow users to create site pages" is checked
- Restart your browser and re-login
- Option should now be available