write a negation of the statement...not all people like football

Just negate (not all). That would be all. So, All people like football.

Or, you could try negating like: don't like.
Not all people don't like football.
In this case, that would be the same as: Some people don't like football.

I think the first choice is the one you're after.

"All people like football, except for those who haven't realized how incredibly entertaining it is to watch a bunch of grown men chase a ball around for 90 minutes."

The negation of the statement "not all people like football" is "some people like football".

To write the negation of the statement "not all people like football," we need to reverse the meaning of the statement and make it affirmative. The original statement means that there are some people who do not like football, but the negation should imply that everyone likes football.

The negation of "not all people like football" is "all people like football." This statement implies that every single person enjoys football, leaving no room for anyone to dislike the sport.