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The error here:
CHAPTER 1.
! Argument of \vspace has an extra }.
<inserted text>
\par
l.84 \chapter{Introduction}

doc class and packages I used:
\documentclass[a4paper,oneside,hidelinks,12pt]{book}
\usepackage[utf8]{vietnam}
\usepackage[english]{babel}
\usepackage{titlesec} % defsc is created ...

Thank you in advance.

This works with my example. Unfortunately, in my whole code, I can not compile when adding that code. Anyway, once again, thank you!

Hi Stefan_K,

Thank you for your help.
I'm new in LaTeX, so there are a lot of things I do not know :)

I did try with google and do as you said, however the result did not change.
The document class that I used here is book, and \titlespacing does not affect \chapter .

I will show my code ...

How can I adjust space between "chapter #" and "title chapter"?

For example, the default is:
Chapter #


Title Chapter

...

and what I want is:
Chapter #
Title Chapter

...