Issues with removing a cartridge from a faucet
That chrome piece with the hex shape on top is definitely a retainer cap which needs to be unscrewed to access the valve/cartridge unit under it. You will need a bigger/better wrench perhaps. I always start by soaking stuck parts with penetrating lube/oil for a while (I use Liquid Wrench but there are a bunch of products out there.
There are specialized plumbing sockets that might allow you to get more leverage on that thin hexed part, like this: