How to insert a blank page in a document ?

It's at Special > Page Break

in older versions*, but all that does is locally override the Pagination of the current paragraph to start at a page top. If you later update formats, removing overrides, it's going to revert.

There are only about a zillion ways to force a new page or an empty page, and this can be further complicated by just how empty you might want a deliberately empty page to be, for example:

• Heading Pagination formats that specify top of page, perhaps top of left/right
• Pagination: Keep with next/previous, that may force content to new page
• Empty anchored Frame-Below that shoves content to new page.
• Empty borderless tables that do the same thing
• Specific paragraph formats that trigger what appears to be an empty page.

PS - Special > Add Disconnected Page
is almost always an unwise way to do it.

* Update: in newer versions, it's on the Insert menu.