You didn’t address my question. If sin is present at the end of the millennium, how could it have ended at the end of the tribulation?
And sin does not end at the end of the Tribulation. Those people who accept Christ during the Tribulation, and survive those seven years, will go into the Millennial reign in their mortal bodies. They will not have received resurrected bodies as of that time and, while they will have received salvation and forgiveness of sin, they will be like people are now - they will have the capacity to sin.
They will also be repopulating the earth, meaning they will be having children and those children will have children who will be born into the world with a sin nature and in need of the same forgiveness and salvation that man has needed since the fall. Some of them will accept Christ's free gift of eternal life, and some won't. There will still be sin during the Millennium, it just won't be as interwoven into every single aspect of life like it is now, it will, for the most part be kept private and hidden, and with Christ in charge of all governments, sin will be dealt with swiftly and severely.
By the time Satan is loosed from hell for a short time to deceive the nations, there will be several generations of still-sinning man on the earth with no memory of the horrors and judgment that sin brought on the earth. There will be many people at that time who, again, like man has since Adam and Eve, choose to reject God in favor of Satan.
So while there will still be sin after the Tribulation and during the Millennium, at the end of the thousand year reign the point will be driven home yet again and for the final time that God is right, that the heart of man is "desperately wicked", and that God was correct all along in how He dealt with sin in providing man a way of reconciliation with Him and forgiveness and freedom from enslavement to sin and the eternal consequences for sin, and His righteous judgment of sin and punishment for sin.