What is a plumbing cleanout and where is it located?
A plumbing cleanout could be located in various locations. Look for a 3-4 inch diameter, circular cap that has a square nut like the one pictured. You can unscrew the lid and your blockage might flow out of that into the yard, instead of inside your home.
As early as the 1990s, most municipalities made it mandatory to install an exterior cleanout at the high end of the drain line, which is typically on the side of the home. If you have a slab-style home, it may be in the front yard.
Plumbing code instructs plumbers & builders to run drain lines beside the water main, with the water main being above the plumbing, to prevent dirty water from seeping into fresh water.
So if you see a hose connection, poke around in that area and you may find the plumbing cleanout.