Here is the sentence again:
Although you might not believe it, Antonio actually benefits from his chicken
scratch handwriting because no one can read
it, Antonio doesn't have to worry about using the right word or the correct
spelling.
Antonio actually benefits from his chicken scratch handwriting is a complete sentence. Because no one can read it, Antonio doesn't have to worry ... is a second complete sentence. Because only blank space exists between handwriting and because, a fused sentence results.
To fix this problem, you could put a period after handwriting and capitalize the B in because.