why i can't download whatsapp on my apple watch?

Hi

WhatsApp Messenger does not currently offer Apple Watch support.

In iTunes, an app that includes an Apple Watch app will state: "Offers Apple Watch App for iPhone".

Look, for example, at Dark Sky (see the left hand column here and/or or look underneath the app name in the App Store on your iPhone):

https://itunes.apple.com/app/dark-sky-hyperlocal-weather/id517329357?mt=8

WhatsApp Messenger does not presently offer this:

https://itunes.apple.com/gb/app/whatsapp-messenger/id310633997?mt=8

You may wish to contact the developer to suggest that they add this feature:

https://www.whatsapp.com/contact/

A convenient way to find other apps for Apple Watch is via the Watch app on your iPhone > Featured tab and/or Search tab (bottom row).