My closing remarks appear on two lines.
\documentclass{letter}
\usepackage[headheight=0pt,headsep=0pt]{geometry}
\setlength{\voffset}{-0.5in}
\usepackage{indentfirst}
\usepackage[USenglish]{babel}
\usepackage[acronym]{glossaries}
\usepackage{amsmath}
\usepackage{hyperref}
\hypersetup{
colorlinks=true,
allcolors=black,
}
\begin{document}
\begin{letter}{Singapore Botanic Gardens\\ 1 Cluny Road\\ Singapore 259569}
\opening{Dear Dr.~Nura Abdul Karim and Ms.~Serena Lee,}
\vspace{0.25cm}
I am looking forward to this unique opportunity. Please feel free to reach out to me with questions or concerns.
\vspace{0.25cm}
\closing{Thank you for your time and consideration,}
\vspace{0.25cm}
\encl{Curriculum vitae}
\end{letter}
\end{document}

\documentclass{...}, the required\usepackage's,\begin{document}, and\end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – Stefan Pinnow Apr 03 '19 at 13:50