P-Trap into Wall Marvel Connector Leaking
What is missing on this setup is the compression nut. It should fit over your 1 1/2 inch  drain pipe going into the wall stub. When you tighten it down the inner gasket will seal around the drain and prevent water and odor from escaping into the room.
This looks like a poor diy job. The problem may have been that the bend in the drain pipe before the Marvel connector prevents the nut from properly threading on the connector. Regardless, you need that compression nut on there. You can pick one up from any home supply or plumbing store. You may have to buy the entire connector. You're also going to have to play with the P trap to get it properly aligned so the nut will thread on. There is play in the pipes. As a last resort you may have to buy a longer 90 degree drain section coming out of the P trap and rotate the P trap 90 degrees toward the front of the sink to get a straight shot into the stub.