The output PDF while using revtex4-2 is in landscape form, how can the problem be resolved?
\documentclass[aps,prstper,preprint,groupedaddress]{revtex4-2}
\usepackage{amsmath}
\usepackage{hyperref}
\usepackage{makecell}
\usepackage{graphicx,lipsum,afterpage,caption,subfigure}
\usepackage{supertabular}
\usepackage{morefloats}
\usepackage{tablefootnote}
\usepackage[final]{pdfpages}
\usepackage{import}
\usepackage{filecontents}
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{siunitx}
\usepackage[inline]{enumitem}
\usepackage{enumitem}
\usepackage{soul}
\usepackage{textcomp}
\usepackage{threeparttablex}
\usepackage{supertabular}
\usepackage{morefloats}
\usepackage{ucs}
\usepackage{color}
\usepackage{array}
\usepackage{calc}
\usepackage{multirow}
\usepackage{hhline}
\usepackage{ifthen}
\usepackage{floatrow}
\begin{document}
\title{Development and Validation}
\end{document}
\usepackage[final]{pdfpages}then the document is no longer landscape. Maybe you have another\usepackage{pdfpages}somewhere? For testing it would be best to use your MWE as you provided in the question (you do need to add some actual text in the body to get some output). When it works in the MWE then you can try it on your real document. – Marijn Apr 03 '20 at 08:57