I don't get how there are people who can't do it.

I don't think it works like that.

Not being able to whistle is a thing?

Nope I can't whistle

I wouldn't expect a potato to whistle at me.

You mean snap your fingers?
My brother tried to teach me, but left a key detail out:

When you have your thumb and middle finger together,
your ring finger and pinky should be pressed down on your palm.

That's what caused me to snap my fingers

I'm able to. I knew that not being able to was a thing, but I didn't know that it was this common.