I just did the same thing. I ran around trying to interact with anything that remotely looked like a coffin. Then I realized I was in the wrong chamber. It's a little confusing that you're in one burial chamber and they tell you to go look for the coffin, but you're supposed to find the coffin in the OTHER family burial chamber. Check your quest log and compare where it states you're supposed to go, and compare that to your map location.
If you STILL cannot interact, it could be a bug, which you can submit using the Help -> Support option on the left menu of this site.