Why can't we work for our salvation if the reward of works is reckoned as debt?
Because a reward earned by works is reckoned as debt, not grace, and no one can put God in debt. Since we are not under a debt economy, salvation cannot be worked for, not even one iota; it is received by believing on the One who justifies the ungodly.
Paul adds that to him that worketh the reward is not reckoned of grace, but of debt. Les explains that we are not under a debt economy. We will never put God in our debt, and so we cannot work for salvation, not one iota. Verse 5 contrasts the one who works not but believes on Him who justifies the ungodly. Abraham was saved by faith plus nothing; he did not have to show works to anybody because he was dealing with God. James looks at it from man's point of view: works show others that faith is real, while God sees faith without works.
Now to him that worketh is the reward not reckoned of grace, but of debt.