There's a remaining patch of adhesive directly under the flash.

Line up the tip of an opening pick with the flash cutout.

Slide the opening pick under the top of the back cover until you feel it start to snag on the adhesive.

You should see the opening pick through the flash cutout.

Keep sliding the pick toward the bottom of the phone until you feel the adhesive completely separate from the back cover.

Avoid touching the rear cameras with your pick. Pushing against the lenses risks damaging them.