-I disagree with slavery
I think you can only disagree with a person. You might want to say "I am opposed to slavery" or "I think slavery is wrong."
-I think that nowadays, in the twenty-first century, it's pathetic that in some countries of this world, slavery still exists as a legal activity.
still, nowadays, 21st century....a bit repetitive
How about "I think it's pathetic that in some countries, slavery still exists as a legal activity." (It's more concise.)
-She could neither be free, nor do whatever/what she wanted.
Seems fine besides that confusion about whatever vs. what.
Check out § 73. whatever. 1. Grammar. The American Heritage Book of English Usage. 1996 for a bit more information.
-I send this story to you
-I send you this story
Usually English-speaking people (at least Americans) use the future or progressive tenses-not the present. So maybe "I am sending you this story."
-I'm sure you'll be against slavery soon
This sentence is a bit random and presumptious.
-You are the only one who can make it possible
Make what possible?
-Your composition is weak in content, you don't explain what you are asked for in the instructions
Are these your teachers comments?
It's a bit unclear but grammatically well written.
-Salma, you were a slave once, (is that) right?
-I paid the owner of the slaves for their freedom. Now they are all free and live in the United States.
Hope this helps!!