Use a flat piece of metal or wood and lay it across the cap, then turn. It will apply pressure to two of the nubs instead of one, preventing damage.

Put some ice and water in a plastic bag and sit it on top of the cap. Let it sit there long enough to cool down the cap.

While the bag is sitting on it, pour hot water on the ground against the pipe to heat up the pipe.

The cool cap will contract while the warm pipe expands. This might loosen it enough to use a straight bar to unscrew the cap.

Edit:

I'm guessing that the real tool would be something like this:

https://www.grainger.com/product/SUPERIOR-TOOL-Drain-Wrench-Rubber-45W758

I assume if it is a solid material, then the standing nubs are for using a spanner bar across them and turning. I haven't seen this specific one before. Sometimes they are depressions and you have a pin spanner wrench.

Sewer cleanout probably. Never seen a cap like this before though, looks kinda flimsy. Maybe get a piece of plywood and cutout/drill for holes that align with the nubs and turn the plywood so you can put force on all four nubs?

Flat blade screw driver and a hammer. Right tight left loose. Press the screw driver against the cap and tap it until it loosens

Take two pieces of wood, longer than the caps diameter. Drill a hole into one of them large enough for a screw. Put screw through and screw it into the other piece of wood, but leave enough space. Lay the contraption on the cap so that two opposing pins are between the woods. Turn screw tight so that the construction grabs those pins tight, like a vise. Turn anti-clockwise to open.

Do not step on it and call the authorities! LOL.