Ephesians 2:9 says salvation is not by works, so that no one can do what?

Answer: Boast

It is by grace you have been saved through faith, the gift of God, not by works, so that no one can boast.

“For it is by grace you have been saved through faith, and this not from yourselves; it is the gift of God, not by works, so that no one can boast.”— Ephesians 2:8-9 (BSB)