Firstly, I don't think you'll ever find a consensus of modern engineers who will all agree. Some modern engineers might say it couldn't be done, but they are selected by the shows to prove the show's narrative. Think of the New York Times, and their "experts" that prove Trump is guilty of whatever.
The biggest single fact that the things were in fact done is that they are there.
Thousands of years old, just standing there, or toppled over by time or whatever--the modern engineer can say it is impossible--but the stone is there nevertheless.
It would better for them to say they don't know how to duplicate it today, not that it couldn't be done (because it WAS)--they don't know--is the final answer.
Ah, but the question posed was "Name one artifact that we couldn’t produce."
You're moving the goalposts. These artifacts can't be produced today with our technology. Of course you can begin a Manhattan-project endeavor if your pockets are deep enough for the billions or trillions of dollars required in order to duplicate artifacts of the type that can't be figured out now. Just try to do it today. Without saying it is possible but we don't currently know how. Of course it was done millennia ago and they exist. That isn't the question. The question is how did the ancients get it done, and what tools did they possess to get it done.