When we sin, we must be punished. The Bible says the punishment for sin is death (Romans 6:23). The death the Bible is talking about is not just our body dying. It means our spirit, which lives forever, will be in everlasting punishment in hell. Jesus explained hell is like a "lake of fire," and that everyone who goes there is separated from God forever and burns in fire that never ends (Mark 9:47, 48). Our sin separates us from God both in this life and also after this life.
We can't have God's plan happen in our lives when we are separated from Him. Our sin comes between us and God. It separates us from His plan for us.
Many things in our lives have gone wrong because of our sin. God still has a plan for us even though we have sinned, but our sin must be forgiven to allow His plan to happen.