Adam and Eve rejected the word of God and accepted the word of the serpent and ate from the tree of the knowledge of good and evil.
God expelled fallen humans from the Garden of Eden to prevent them from eating the fruit of the tree of life and living eternally.
[Related Verse]
Genesis 3:22
And the Lord God said, Behold, the man has become like one of us, knowing good and evil.
Genesis 3:23
The Lord God sent the man out of the Garden of Eden to plow the land of his origin.
Genesis 3:24
So God drove the man out, and at the east of the Garden of Eden he placed the cherubim and a flaming sword that turned every way to guard the way to the tree of life.