3 way switch problem
The screws on a 3 way switch are named traveler, traveler, common, and ground. You need to connect the wire that used to be on the dimmer common screw or wire to the rocker switch common screw. The common screw is usually darker colored. If the wires to the two traveler screws are reversed, that causes no problem. Sometimes there is no ground screw which should be green colored in which case a ground wire should be fastened securely against the switch's metal bracket where it is screwed onto the box in the wall.
The locations of the screws vary from one switch to another so matching up left or end or bottom is not always the correct way to connect things.