***Solution found thanks to Neo001***
Like many others, I've been unable to use Google Assistant to send a WhatsApp message. The response is always the same, that there is no WhatsApp number assigned to the contact.I believe I may have spotted the issue, the Samsung Contacts app does not list the WhatsApp numbers in the same way as the Google Contacts app does, i.e. the Google Contacts app will show all numbers for WhatsApp actions available such as sending a message / making a call / making a video call.
It maybe the Samsung Contacts app needs to be updated to list the numbers for WhatsApp on each contact. Alternatively, Samsung should give us the option to use the Google Contacts app as a default as this is not an option at this time.
