How to get rid of lots of natbib warning?

Question

Again when I run my main.tex file, LateX prompts up with number of lines which has following pattern:

Package natbib Warning: Citation `piaget1973understand` on page 47 undefined on input line 44.

Underfull \hbox (badness 1000) in paragraph at lines 44--45

and at the end:

Appendix A (./mainchapters/appendix.tex) (./main.bbl [49] ./main.bbl:43:Missing  $ inserted.
<inserted text>
$
l.43 ...s?id=com.google.android.apps.seekh\&hl=en_us.
?

main.tex

documentclass[12pt]{report}

\usepackage{hyperref}
\usepackage{natbib}
\usepackage{enumitem}
\usepackage[utf8]{inputenc}
\usepackage{csquotes}
\usepackage{graphicx}
\graphicspath{ {images/} }
\usepackage{caption}
\usepackage{subcaption}
\usepackage[a4paper, width=150mm, top=25mm, bottom=25mm, bindingoffset=6mm]{geometry}
\usepackage{ragged2e}


\date{02 March 2020}

\begin{document}

\input{titlepage}


\chapter*{Dedication}

To my parents.

\chapter*{Declaration}
I declare that...

\chapter*{Acknowledgement}
I want to thank...

\tableofcontents

\listoffigures

\chapter*{Abstract}
\input{chapters/abstract_v2}

\chapter*{Introduction}
\input{chapters/introduction_v2}

\chapter{Children Computers and Constructionism}
\input{chapters/chapter1_v1}

\chapter{Design methodologies for children's technology}
\input{chapters/chapter2_v1}

\chapter{Toys of 21st Century}
\input{chapters/chapter3_v1}

\chapter{Child Computer Interaction in Developing countries}
\input{chapters/chapter4_v1}

\chapter{Current scenario}
\input{chapters/chapter5_v1}

\appendix
\chapter{Appendix Title}
\input{mainchapters/appendix}

\bibliographystyle{apa}
\bibliography{/Users/pythonbuddha/Documents/Thesis/References.bib}
\end{document}