21:15a Then God sent an angel to destroy Jerusalem,

⌊During the epidemic⌋ he/Yahweh sent an angel to destroy Jerusalem.
-or-
God sent an angel to kill the people in Jerusalem.

21:15b but as the angel was doing so,

But then he saw the angel start to kill people. (Easy English Bible)
-or-
But as the angel started to kill people, (EEB)

21:15c the LORD saw it and relented from the calamity,

Yahweh saw what was happening. He decided to stop the punishment that he was causing, because he felt sorry.
-or-
Yahweh saw the suffering that the people endured. He had mercy on them and stopped the punishment.

21:15d and He said to the angel who was destroying the people,

So he said to the angel, who killed the people,
-or-
So he spoke to the angel who was destroying the people. (New International Reader’s Version)

21:15e “Enough! Withdraw your hand now!”

“Stop what you are doing! That is enough!” (Translation for Translators)
-or-

⌊He/Yahweh said that the angel⌋ should stop and not kill any more people.

21:15f At that time the angel of the LORD was standing by the threshing floor of Ornan the Jebusite.

This happened when the angel of the LORD was standing where Araunah/Ornan the Jebusite threshed his grain.
-or-
Yahweh’s angel was at that moment standing near the place where Araunah/Ornan used to beat the straw to gather the grain. ⌊Ornan was from⌋ Jebus ⌊which is the old name that the city of Jerusalem had before David conquered it⌋ .