Using Surface Pro 4 Pen Mask Brush Shift-Click Issue

I have a Surface Pro 4 with an i7 processor. I was trying to use the subtract brush on a radial filter mask to create straight lines using Shift-Click Select with the pen. I've replace the original pan with a Metapen. The pointer shows a negative sign and the word SHIFT inside a circle all the time. I can't clear it or reset it to create a new starting point to create a straight brush line. Am I missing something obvious? Any ideas on how to use a pen successfully in Lightroom Classic?