Mac Pro (Early 2015) El Capitan Ver 10.11.4 Factory Reset Issue

I want to restore my mac to factory settings. I backed up my stuff and followed the steps provided by Apple (What to do before selling or giving away your Mac - Apple Support). After holding down command+R and the power button, I reached the erase window. However, on the window, my computer's hard drive is lightly colored and I cannot select my hard drive to erase it. Why is this happening & how should I fix this?