Here are some things you can do:
1. Turn on "soft springs" in the study options. Run that.
2. Turn on "inertial relief" in the study options. Click "YES" when the warning about force imbalance pops up. Show the displacement plot UNdeformed and you should see the culprit.
3. RMB on the mesh and click "Show Mesh." Give it a good look-over to make sure there are no parts hanging off in space by themselves.
4. Apply "bonded" contact sets between things that should be stuck together, but you've modeled them with a gap.
5. Turn OFF inertial relief when you've got everything working nicely. Then re-run.
Thanks for these tips David. Was going nuts trying to figure out why my model wasn't sufficiently constrained. Once I used your steps, I found 2 bodies that were only connected by their corners.
learned this from our ttm brian zias, a frequency study if you have simulation pro is a great way to find parts that aren't connected