I need your expertise for something ;) I would like to have the "Chapitre" vertically centered compared to the number of the chapter. Here's what it is looking right now :

This is the code I use to make it look like on the picture :

\documentclass[11pt, a4paper,oneside]{book}
\usepackage{titlesec}
\titleformat{\chapter}[display]
{\normalfont\Large\centering}
\fontsize{150}{60}\selectfont\color{gray}
{\fontfamily{Domitian-TLF}\selectfont \bfseries\thechapter}}{15pt}{\bfseries\Huge}

(The number "1" of the chapter use the "Domitian" font).

After I completed your MWE by adding \begin{document} chapter title and text \end{documernt} it only compiled with erros such as \color being undefined and problem with the \fontsize{150}{60} not available and size <24.88> being substituted.

However, you can raise the \chaptername typesetting like this:

% chaptitleprob.tex  SE 567408

\documentclass[11pt, a4paper,oneside]{book}
\usepackage{titlesec}