Why am I unable to drag & drop to Case Detail area? (I get a red circle with diagonal line)

From the options in the pane at the top of the page, hold CTRL/command, then click and drag Product, Type, and Case Reason down to the Case Detail section below. (Note: There are two Product fields; hover over them and select the Picklist field, not the Lookup.) I am NOT ABLE TO DRAG & DROP inside the Case Detail area. WHY? I'm not able to move forward.

URL: https://trailhead.salesforce.com/content/learn/projects/set-up-case-escalation-entitlements/create-support-processes-cases