I would also do a acetone soak and use a wooden toothpick to see if that area lifts off. I too think it looks like tape.
Tape was my first thought as well, but with my magnifying glasses it's shiny and definitely looks metallic. I would hate to remove it if I'm right. Would the acetone soak it off if it's a piece of foil or metal?
That is definitely not a struck through error. If you think it's an error then send it to NGC or PCGS for attribution. Let's us know what they tell you
It may look like something. We don't have the coin in hand. But if it dissolves or removes, glue, tape etc. it's nothing. If it is a retained strike through, the metal should stay attached to the coin. I'd let it soak in a glass jar with a proper lid, with a little acetone to cover completely for (a couple days?)