Our inflatable stand up paddle boards ship with a small ladder-shaped protector inside the fin box, to help protect it during shipment. It needs to be removed before the fin can be installed.
Take the paddle board valve wrench included in your package, put the handle end in to one of the open areas give give it a nudge. After a few times it should move but if not, try hitting it further into the fin box then try the correct way to get it out. This may loosen it faster. Using a tiny amount of cooking spray or oil may help as well.
Do not ever use anything sharp on your board as you do not want to puncture it, this is why we would recommend the handle of the valve wrench or something blunt like a wooden spoon.
Here is how to insert the fin: