Can someone help?
Laundry Outlet Box comes with a plug in the outlet for 2 reasons.
First, this type of box can be used with the supply lines in the box or turned over and use without the supply line openings.
Second they come with a plug for when you are doing new construction and the system need to be wet tested for leaks.
The plug can easily be removed with a small screw driver, hammer. A pair of pliers is also recommended to keep the plug from falling into the pipe.
Check out this video for help on getting this done.