"Cannot execute boolean operation"

I’m trying to use the boolean difference modifier to remove one object from another. As soon as I select “boolean” the modifier label in the sidebar has a red background and when I try to use it says “Cannot execute boolean operation”. Can anyone think of what might be happening? Thanks in advance.

Without a file to examine, we can’t guess.

True! I don’t post here much and wasn’t familiar with how this works, but I could have guessed that. It’s attached to this post.phneergewallet4.blend (1.15 MB)
Note that I already tried simplifying the model with the decimate modifier and removing doubled vertices in edit mode. Neither helped.

It’s attached (I thought I’d already done that). phneergewallet4.blend (1.15 MB)

This is odd. I tried doing the exact same thing with the Windows version of blender and it worked. So problem solved, for some reason…