Sending Whatsapp message through Automator
First you should check if the Whatsapp is also enabled on Privacy Settings. Plus you can try doing that using a Run Applescript action. Something like this:

on run {input, paremeters}
     tell application "Whatsapp"
          activate
     end tell
     tell application "System Events"
          keystroke "Your message here"
     end tell
end run


Assuming you have the right group/person window already selected.