Idk if you already found a solution to your case, but I had the same issue.
In my specific case the blueprint actor I was using for some reason ! didn’t have a mass ! and thus physics could not be simulated (which kinda makes sense when you think about it).
I had this piece of paper, which is just a 2D object.
Since all materials are one sided (backface culling) by default in most 3D programs, I changed the material to “Two Sided” for the material of my static mesh. I changed a bunch of other stuff and I seriously doubt that that was the reason why it suddenly didn’t allow me to tick “Simulate Physics”, but there you go.
I then noticed that when I clicked on the Static Mesh component of my Actor Blueprint, that it had no Mass.
So I clicked on Mass and set it to 0.01 kg. Thanks to that, simulate physics was no longer greyed out.
The weird thing is that once I unchecked mass again, Simulate Physics was still set to true.
Frankly, I am as confused as you as I type this, since I though it was the mass that was the problem, but now I realised that I can set simulate physics regardless. But I’m pretty sure that was what I changed before it started working again. This is for sure weird.