If the pipe is threaded, you can use a threaded plug. If not, you can get the plug type that has a rubber piece that expands out to seal as the nut on top is tightened (that is what your current plug appears to have been but is now missing the actual plug part). Either way, do it immediately since you currently have sewer gases entering your home - not good. Home Depot, Lowes, or any plumbing supply store should care what you need.

Alice suggests a good temporary quick fix by inserting the proper sized test plug to stop the gasses from stinking up your room. From what I could determine, the pipe is plastic. You could glue in a sleeve with a threaded cap in it.

Thank you very much for your help. I ended up buying "CherneÂs Gripper plug" and it seems to fit tightly.

I also bought a threaded sleeve/insert that fit snug, but could be pulled out fairly easily so I took it back. Is it possible too use PVC primer/cement to install the sleeve (then use the threaded insert with teflon tape)?

Careful here. That cleanout is in an open area in the floor. If you install an expanding test plug as was suggested above the threaded rod and nut on the plug would extend slightly above the floor surface and present a trip hazard. The same would be true if you installed a threaded sleeve and a conventional cleanout plug. In this case the Nut on the cleanout cap would also extend upwards above the floor, creating a trip hazard.

The proper solution would be to cut the pipe flush with the floor, install a threaded sleeve flush with the floor, then install a cleanout cap with a recessed square wrench opening to insure there is nothing protruding above the floor line.