Mary acknowledges she needed a Savior.
She was acknowledging her sin.
There's no mention of Isaiah sinning in the Bible either. Doesn't mean he was conceived immaculately.
Mary was conceived without sin, and chosen at the beginning of time to be the Mother of Christ. Her sin was forgiven, thus, before she was born.
God can do all things. It’s called the Immaculate Conception.
Do some studying on it and you will find out that this is the truth.