What Does This Mean? - "rigid body can't be child of a non compound rigid body"

I was rendering an older blend file I made in Blender 2.79 (a rigid body tank animation), and noticed that the rigid body cannon ball that fired from the tank’s cannon (another rigid body that the ball is supposed to be parented to) gave me this message when looking at it’s physics properties.

The animation works fine in 2.93, except that the ball remains stationary in front of the cannon after firing. The ball still activates the target (a rigid body wall held together with constraints) in spite of staying where it ends up a few feet beyond the cannon. Very weird.

Anyone know what’s going on and how to fix it? Blend file is way to big to find a place to upload for sharing.