I suspect it is maybe Luke's original Jedi temple, but that's my baseless speculation. Do the novelization, script or any other resource offer further insight into this scene?
WGA script says this:
She turns to look -- we PIVOT -- and see a BURNING TEMPLE AT NIGHT. We PAN to:
R2-D2 -- who watches the flames -- and a MAN appears (LUKE, whose face we do not see). He falls to his knees, reaches out to the droid -- with a MECHANICAL RIGHT HAND. We PUSH IN ON REY as RAIN BEGINS -- and DAY TURNS TO NIGHT -- and she LOOKS UP -- we TILT UP --
Note that the burning Temple is in an earlier scene, before Luke and R2D2 show up and definitely before the KoR.
To see we're LOOKING UP AT A WARRIOR as he is STABBED BY A FIERY LIGHTSABER! He screams and falls to the ground -- we FOLLOW HIM, revealing Rey again, now in a nighttime battlefield. She gets to her feet, frightened by what she sees.
We PIVOT AROUND HER to REVEAL KYLO REN, and the six other KNIGHTS OF REN, who flank him!
In other words, the script says nothing useful at all about KoR background. But it seems like it's probably NOT Luke's destroyed Temple.
Interestingly, the Foster novelization reversed the order and mentions SOME Temple as background. No idea if it was Luke's temple; but Luke and R2D2 were in a next, SEPARATE scene in the book.
Day became night, sky ominous and filled with rain, cold and chilling to the bone. She was standing, she was sitting, she was looking up—to see someone, a warrior, take the full force of the lightsaber. He screamed and fell.
Battlefield then, all around her. Putting a hand to her mouth, she rose and turned. As she turned, she found herself confronted by seven tall, cloaked figures, dark and foreboding, all armed. Soaked and shivering, she stumbled backward, turning as she half fell. Firelight illuminated her, firelight from a distant, burning temple.
The seven vanished. A sound made her turn, and she blinked in surprise at the sight of a small blue-and-silver R2 unit.