I have trouble in using the cas-sc LaTeX template when my manuscript was sent back. The editor replied:
Type the whole manuscript with double line spacing.
I referenced Campa's answer and added \doublespacing after \maketitle and I also added \doublespacing after \keywords. but no matter what I do, the ABSTRACT cannot be double spaced. The following code is the minimum samples for the problem.
\documentclass[a4paper,fleqn]{cas-sc}
\usepackage[square,sort,comma,numbers]{natbib}
\let\printorcid\relax
\usepackage{lscape}
\usepackage{setspace}
\ExplSyntaxOn
\keys_set:nn { stm / mktitle } { nologo }
\ExplSyntaxOff
\begin{document}
\let\WriteBookmarks\relax
\def\floatpagepagefraction{1}
\def\textpagefraction{.001}
% Main title of the paper
\title [mode = title]{This is title}
%%%%%%% second author %%%%%%%
\author[1]{A person}
\ead{aperson@outlook.com}
\credit{}
%%%%%%% Corresponding author %%%%%%%
\cormark[1]
\cortext[1]{Corresponding author.}
%%%%%%% affiliations %%%%%%%
\affiliation[1]{organization={organization},
city={city},
postcode={000000},
country={country}}
% Here goes the abstract
\begin{abstract}
this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract this is abstract
\end{abstract}
\begin{keywords}
\doublespacing
\sep keywords
\sep keywords
\sep keywords
\sep keywords
\sep keywords
\end{keywords}
\maketitle
\doublespacing
\section{Introduction}
this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro this is intro
\end{document}
cas-sc.cls can be downloaded here

els-cassuite is essentially a collection of bugs. It's hard to understand why a journal wants you to follow some specification that the class they force you to use doesn't offer. – egreg Mar 13 '24 at 12:33