If God is sovereign and has a plan for all things (Ephesians 1:11), how can humans truly have free will?

You are not a robot. Your choices really do matter and have consequences.
But God is still sovereign and will bring about the ends that he has predestined to occur from the beginning.

As Acts 2:22-23 explains: “Men of Israel, hear these words: Jesus of Nazareth, a man attested to you by God with mighty works and wonders and signs that God did through him in your midst, as you yourselves know— this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men.”

Killing Jesus was a sin and God never causes sin (James 1:13). So people freely chose to crucify him.

But who planned for the crucifixion to happen before humanity even began? God.

Therefore, God is so sovereign that he can bring about his will without violating the free will of humans.