I am using twentysixteen WordPress theme, Whenever i tried to split long post in multiple page by tag nothing happened. Kindly help me.
You have to add <!--nextpage--> in Text mode of editor. If you add it through Visual editor - WP will escape it to HTML:
and display it literally (instead of pagination):
So, check your post by switching to Text editor. If nothing isn't displayed at all when you view the post, this may be a plugin issue. Update your theme to the latest version and disable all plugins temporarily.