To say ‘no’ to God means to sin, to go against the will of God.
Mary was Immaculate, born without sin, and never in her life commited sin. Therefore never in her life did she say no to God.
You talk about her free will. Well, one is trully free only when one chooses to walk towards the light. To choose darkness is enslaving (you can’t see where you are going; you can stumble, fall, etc).
She had free will, so she freely chose to fulfill God’s will.
What reasons could she have had to say no? Selfishness?, fear?, that she did not care about her people having a Saviour? All those reasons would be sinful, all of them would come frome someone that is not totally open to the grace of God. Remember Mary was called by the Angel: ‘full of grace’; she was full of grace, in total sync, so to speak, with God.
So, she said yes and we bless her for that!!!

:harp:
Alma