Bassinet/pack&play mattress

So my son sleeps in the top portion of his pack n play with the mattress that comes with it... I’ve noticed that it has a crease from the mattress being able to fold which makes it uncomfortable for him and he hates sleeping in it ( so we usually position him to where he’s not laying on it).

I also have a bassinet that we keep in the living room & it also has a crease down the middle bcuz it can fold in half..

How do I get rid of this?

I’ve read that it should be completely flat.