The drain line trap for the wash machine may be in the wall below the water supply faucets or under the floor. No matter what its position, the first effort should be to clear the clog with a snake. Since the is usually a larger 1 1/2" line this will be fairly easy.
If for some reason you have to get access to a trap hidden in the wall with no access panel you are going have to cut or remove the panel and repair or replace it later. If the trap is located under the house you will have to cut through the belly material, do your work and then repair the belly.