Autumn dusk is the most beautiful season, and it is a sick sentence, which is not properly matched. Analyzing the main sentence "Twilight is a season", we can see that the subject and object are not properly matched, and delete "Twilight".
Ill sentences refer to sentences with grammatical, rhetorical or logical defects. Any sentence that violates the rules of grammatical structure or objective facts is a sick sentence. The former is called grammatical error and the latter is called logical error.