YOU SAID:

she could not come to the party because she was very busy

INTO JAPANESE

彼女はとても忙しかったのでパーティーに来られなかった

BACK INTO ENGLISH

She was so busy that she could not come to the party

INTO JAPANESE

彼女はとても忙しかったのでパーティーに来られなかった

BACK INTO ENGLISH

She was so busy that she could not come to the party