With iOS 10 Non-Apple-Developers got the possibility to use Siri for their apps. One of the first apps using this new possibility was WhatsApp. Now you can ask Siri to send WhatsApp messages or start WhatsApp calls for you. So, asking Siri “Send a WhatsApp message to CONTACT” will now do exactly that.
So normally if you would say “Send a WhatsApp message to CONTACT”, Siri would just ask for the content of the message. But some users just get the answer: “Sorry, you’ll need to continue in the App”. But even if you tap “open WhatsApp”, nothing happens, except that WhatsApp opens. But even if WhatsApp is open, Siri still doesn’t want to send the message. The problem is that if you updated WhatsApp first and then just updated iOS to iOS 10 you will have this problem.