Apple watch

That's different from WhatsApp not being available for Apple Watch. Where are you double tapping? You would double tap the elongated power button on the side of the watch or the power button on the side of your iPhone if downloading on your iPhone.

See this link for more information on how to download apps on your watch or iPhone --> Download apps on your Apple Watch - Apple Support

WhatsApp is not an App which Facebook (Meta, the parent company) has released for Apple Watch. Since this is NOT an Apple owned company or app, you need to contact Facebook and let them know this is something you'd like to see. Apple cannot bring the app to Apple Watch.