How can I center this cell text?
\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage[left=1.27cm,right=1.27cm,top=0.7cm,bottom=1.3cm]{geometry}
\usepackage{fancyhdr}
\usepackage{array,multirow,makecell}
\usepackage{colortbl}
\usepackage{xcolor}
\usepackage{here}
\newcolumntype{R}[1]{>{\raggedleft\arraybackslash }b{#1}}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash }b{#1}}
\newcolumntype{C}[1]{>{\centering\arraybackslash }b{#1}}
\pagestyle{fancy}
% pour les pieds de page
\cfoot{ \textsubscript{ Trame du document : FOR\_TEMPLATE }}
\rfoot{\textsubscript{ \color{red} Ne pas reproduire / Copie contrôlée}}
\begin{document}
\begin{table}[!h]
\centering
\begin{tabular}{|C{3cm}|L{13.9cm}|}
\hline
\rowcolor[RGB]{33,88,104}\multicolumn{2}{|c|}{\textbf{\color{white} Abbreviation }} \\ \hline
\textbf{Abbreviation} & \textbf{Meaning} \\ \hline
COO & Chief Operating Officer \\ \hline
RQ \& AR & Regulatory Affairs and Quality Manager \\ \hline
ANSM & Agence Nationale de Sécurité du Médicament et des produits de santé (National drug and medical devices agency) \\ \hline
CPP & Comité de Protection des Personnes (Ethics Committee) \\ \hline
CNIL & Commission Nationale Informatique et Liberté (French data protection agency) \\ \hline
CER & Clinical Evaluation Report \\ \hline
IFU & Instructions For Use \\ \hline
NC & Non-conformity \\ \hline
PMSR & Post-Market Surveillance Report \\ \hline
PSUR & Periodic Safety Update Report \\ \hline
CAPA & Corrective and Preventive Actions \\ \hline
\end{tabular}
\end{table}
\end{document}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash }b{#1}}
to\newcolumntype{L}[1]{>{\raggedright\arraybackslash }m{#1}}
(and possibly the same for the other column types), but is there a reason you had "b" there to begin with?