16 It does not, therefore, depend on human desire or effort, but on God’s mercy.

Read Romans 9:16 Using Other Translations

So then it is not of him that willeth, nor of him that runneth, but of God that sheweth mercy.
So then it depends not on human will or exertion, but on God, who has mercy.
So it is God who decides to show mercy. We can neither choose it nor work for it.