Can someone explain why "Ti lidé dělají to špatně" is not accepted?
The second position rule for "to". It is a clitic here.
My corrected answer says “Ti lidi to...”, but the answer on this discussion page says “Ti lidé to...”. Which is correct - “lidé” or “lidi”?
Both, but lidi is informal.
Your answer is currently accepted. At least the part you show. So I don't know what was wrong, we really need to see the complete and exact sentence.