Augustine described the various states of mankind thus:

• Before the fall, Adam and Eve were in a state of posse peccare (able to sin) and posse non peccare (able not to sin)
• After the fall, all of mankind is in bondage to sin, a state of non posse non peccare (unable not to sin)
• After death, the redeemed will be in a state of non posse peccare (unable to sin)

But why can Christians no longer sin in Heaven? GotQuestions addresses this issue here,

“The Bible’s teaching is that heaven or the eternal state is completely holy. There will be no possibility of sin, we will be clothed with righteousness (Revelation 19:8), and we will be eternally confirmed in our state of bliss. The work that God promised to complete in us will have been finished (Philippians 1:6). Our deliverance will be complete, as the elect are redeemed—body, soul, and mind—to the glory of the Lamb (Revelation 5:6–10).“

Christians can no longer sin in eternity because they choose to love and follow God in this life – and after death, their will is then fixed by God irretrievably towards good. Conversely, those that reject God in this life will have their will fixed after death irretrievably towards evil. Think of it this way: your will is locked by God after death based on the choices you make in life. Two doors lay before us with each one opening to starkly different eternal destinies. In John 10:9, Jesus said “I am the door. If anyone enters by me, he will be saved and will go in and out and find pasture.” Once we go through the door of our choice in death, it locks behind us for all eternity.

Here’s the frightening thing: many foolishly think they can be “neutral” and simply not choose either door. But Scripture is very clear that not choosing the door of life, means one has already chosen the door of death. Jesus warns in Matthew 12:20 “Whoever is not with me is against me, and whoever does not gather with me scatters.”