Is using the definite article here in this sentence incorrect? If yes, what's its reason? The word 'chapter' is a count noun.
Have you finished the chapter eight of your novel book?
No, you can't say:
Have you finished the chapter eight of your novel?
You can say:
Have you finished chapter eight of your novel?
Have you finished the eighth chapter of your novel?
Have you finished eight chapters of your novel?
Have you finished the first eight chapters of your novel?
(Some of those differ slightly in meaning, but I include them for comparison.)
"The eighth chapter", "the fifth book", "the second volume", "the fourth page", "the second series" take the definite article. But "chapter eight", "book five", "volume two", "page four", "series two" don't.
You could also have a table in a restaurant called "table 7", or an airport terminal called "terminal 2", a runway called "runway 1", or a gate called "gate 9".
The numeral after the noun causes it to refer to a specific chapter (etc) and causes the definite article to become impossible.