The slide-in fins can occasionally become stiff to install and remove due to the fin box being slightly curled when stored.
Normally, the fin can be removed or fitted by gently heating the fin box up with a hair dryer for a few minutes which makes it more supple and should allow the harder fin plastic to slide in and out. Also, if you are hitting the fin to remove it, please do so by bracing a block of wood or similar against it and hitting that, not the fin directly.
Once you have got the fin freely moving in and out of the box after heating it, leave the fin in place and the tail of the board flat for a few days and this should set a new shape to the fin box and make future fin fitting and removal easier.
This video shows how you can resolve this: