iPad's "Markup" feature's "ruler" is not working.

Why will the ruler not work? The device simply won't recognize it. I have an iPad (6th Generation) running 15.1 along with an Apple Pencil 1.0. It was working just three days ago, and when I went to use it again today to mark up another PDF, it doesn't work. I've searched previous questions like this and the common answer is "Try updating it." I did, as I was at the time using 14.3 and that had no effect. 15.1 is just the same. (And, to be honest..."try updating it" is a very unhelpful answer anyways).

Is there any TRUE fix? I've restarted it. I've let my pencil drain it's battery to 0% to try to see if it's the problem. I just don't understand. And yes, I've tried using just my finger. Same result.

I get so frustrated when technology doesn't work like it is designed to, especially when it is a simple feature that worked previously!