Q: I am a iphone 6S  user. I had noticed that the are no KEYNOTE, PAGES, NUMBERS

Get Apple apps for free on your iPhone, iPad, and iPod touch - Apple Support

I think the key is the September 2013 date. "You can download some Apple apps for free if you activated or purchased an iPhone, iPad, or iPod touch with iOS 9 or later on or after September 1, 2013."

See also this person who solved his own question:

Pages not free?!

Contact Apple.  See if you find your product using this link if you are in the US

https://getsupport.apple.com/

Were you the first owner of this iPhone or did you buy it used?  The free apps are only available with first activation of a new device. Once that activation is used, only the person who owns the AppleID used with that activation can re-download the apps for free. Any subsequent activation by someone else with a different AppleID is not eligible for free downloads of those apps.

From what seller in what country did you purchase the iPhone?

On what date? Do you have proof of purchase showing serial

number of the iPhone?