Free will is the ability of people to make choices different than God's commands.
A sin is a thought or act contrary to God's will for us.
Without the ability to sin, there is no free will.
Our problem is that free will is never addressed in the Bible. Bob DeWaay describes it this way:
Even in passages where prophets and others asked God why He allowed so much evil to harm the innocent, it was not discussed. The answer was never that God was committed to the principle of free will and determined that allowing evil was a necessary by-product of free will.
Most free will theology is based on philosophical considerations that are imported to the discussion from outside the Bible. Since the Bible does not directly discuss the meaning of “free will,” the concept must be derived from passages about human bondage to sin and human responsibility and culpability before the Law of God. You will see this as we examine literature on the topic.
Free will is not the simple answer to important theological questions that people think it is. It raises more questions and complications than it answers. I set about to study this matter in great detail over ten years ago. I read the best material I could find, much of it sited in this article. The bottom line for me is that we need to accept what the Bible teaches and not try to escape from clear Biblical passages through philosophical speculation. I am not minimizing the sincere desire people have to answer the difficult question about God’s relationship to time, evil, and human choices. But I am saying that outside of Divine revelation in Scripture there are true mysteries.
it's worth the time to read all of Bob DeWaay's essay.
My personal opinion is that God considers the free will of man to be the prime directive, with everything else supporting free will. But my opinion is not supported by the Bible.