I am not aware that Duolingo has a Mac app. The HTTP 406 error means the requested content cannot be rendered on the device, and is little more than a clue that you're attempting to do something the app cannot provide.
Try these suggestions: If Safari doesn't open a page or work as expected on your Mac - Apple Support. In addition to them, if you are using any non-Apple "anti-virus", "cleaning", or "Internet security" products including "banking security", anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do. And if you're using a VPN, try not using it.
Other than those suggestions you will need to contact its developer for assistance. It would appear your Duolingo account is ok, but there is something else preventing you from using their website on your Mac.