Because she was sick.
The mistake is that is not a sentence. It is a sentence fragment.
You need to have (Because she was sick), what.
She could not go to school because she was sick.
Because she was sick, she could not go to school.
She had to go to the doctor because she was sick.
Because she was sick, she had to go to the doctor.
She missed the field trip because she was sick.
Because she was sick, she missed the field trip
"Because she was sick" is an incomplete sentence.
"Because she was sick" is not any type of sentence structure. It is called a sentence fragment.
I am sickened by you, you fat git
a fragment
That was not a mistake!
I had to call in sick to work today because I have the flu.
You can say "I am sick of the recession" or "Jaime had to stay home from school today because she was sick with the flu."
Because of a mistake in its address, your letter did not arrive until january 3.
The sentence "As I was sick, therefore I could not go there" is grammatically correct, but the use of both "as" and "therefore" is redundant. You could say, "I was sick, so I could not go there" or "Because I was sick, I could not go there."
This sentence is a declarative sentence because it is making a statement.
i was absent yesterday because I was sick that day
I needed a representative to do my presentaion because I was sick.