\textbf is not working (in Vietnamese)

I'm using kaobook to write a book in Vietnamese and I have a problem that the commands \textbf, \textsc, etc are not working.

\usepackage[utf8]{inputenc} \usepackage[vietnamese]{babel}

The above commands are used so that I can write Vietnamese, but the chapter and section titles are not bold.

I need your help.

Hi, thanks for reaching out. I would suggest to use xelatex or lualatex instead of pdflatex. In that case, you don't need the inputenc package, and you'd use polyglossia instead of babel. Also make sure that the bold fonts are installed on your computer.