To fix your model, try the following.

Delete the Fixed Joint to ground.

Use Fixed Supports on the four vertices that are on ground.

Edit the Fixed Joint to surface and replace the surface with the six edges of the holes. Try to solve that. If it doesn't work, delete that joint and create a Fixed Joint between the edge of one hole and the beam vertex in that hole. Repeat for the other five holes/vertices to get six fixed joints on the disk.