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
That didn't even make that much sense in English.