The pages and publisher attribute are not shown from the BibTeX.
elsarticle-template-num.tex file:
\documentclass[preprint,12pt]{elsarticle}
\usepackage{xurl}
\usepackage{amssymb}
\journal{Nuclear Physics B}
\begin{document}
\begin{frontmatter}
\section{Introduction}
several other countries\cite{R1}.
BibTeX file :
@techreport{R1,
author = {Organization, World Health},
mendeley-groups = {Covid 19 Chest Xray},
pages = {9 p.},
publisher = {World Health Organization},
title = {{Coronavirus disease 2019 (COVID-19): situation report, 51}},
type = {Technical documents}
}

@techreportnormally does not have apublisher. Instead it has a field calledinstitutionthat plays a similar role. – moewe Mar 03 '21 at 16:31author = {Organization, World Health},is wrong. "Corporate authors" must be protected with an extra pair of curly braces:author = {{World Health Organization}},. See https://tex.stackexchange.com/q/10808/35864. Also I would probably drop theinstitutionif it coincides with theauthorin this case. – moewe Mar 03 '21 at 16:32@techreportrefers to a single work, you do not need a page number to find the work itself. The BibTeX standard styles and also theelsarticlestyles simply do not have a field for the number of pages here (which is usually not very relevant anyway). If it is so important to you that you need to show it, put it into thenotefield. – moewe Mar 04 '21 at 06:04