How can I download WhatsApp on my iPhone?

This is described in the article Download apps and games on your iPhone or iPad - Apple Support

Note that not all apps are available in all countries.

If an app or feature on your device is not working, it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution. See how to use Screen Time on an iPhone, iPad, or iPod touch by clicking here --> Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support