It is the part of the page. I have read we can't use a negative sentence with 'should' if the sentence we construct is refering to should (obligation).
Could you please check the page? Because if you do it, you will see the page written ''NO NEGATIVE FORMS".
Yes, I've looked at the page. I don't completely agree.
"I should be at work before 9:00."
I don't understand why they wouldn't accept "Shouldn't you be at work before 9:00?" as a negative question with same base meaning. They might argue that this changes it from a "supposed to" form to an "ought to" form. The nuances there are too subtle for me to agree with.
What they might be trying to say (and this is consistent with their other examples) is that you can't say "I shouldn't be at work before 9:00" to mean the negative of "I should be ..." It's true that if you wanted to say "I don't need to be at work before 9:00", ie. the opposite of "I should be ..." there is no appropriate form using "should".