[Tex/LaTex] Which package is needed for \boldsymbol{}

Question

I am using \boldsymbol{} and \textit{}, but they are not recognized. The document preamble is the following:

\documentclass{book}
\usepackage[spanish]{babel}
\selectlanguage{spanish}
\usepackage[utf8]{inputenc}
\setcounter{tocdepth}{4}
\setcounter{secnumdepth}{4}
\usepackage{parskip}
\usepackage{graphicx}
\usepackage{multirow}
\usepackage{amsmath, calc}
\usepackage[margin=1.5in]{geometryit is}
\usepackage{hyperref}
\usepackage{textcomp}
\usepackage{listings}
\usepackage[svgnames]{xcolor}
\usepackage{fontspec}
\usepackage{mathptmx}
\usepackage[nottoc,notlot,notlof]{tocbibind}
\usepackage[numbers]{natbib}
\begin{document}

For example, I'm using $\boldsymbol{x}_t$ and 
\textit{vector de estados} within a paragraph.

\end{document}

Which lines concern the font or the commands that are not working? What should I change?

Answer 1

\documentclass{book}
\usepackage[spanish]{babel}
\selectlanguage{spanish}
\usepackage[utf8]{inputenc}
\setcounter{tocdepth}{4}
\setcounter{secnumdepth}{4}
\usepackage{parskip}
\usepackage{graphicx}
\usepackage{multirow}
\usepackage{amsmath, calc}
\usepackage[margin=1.5in]{geometry}
\usepackage{hyperref}
\usepackage{textcomp}
\usepackage{listings}
\usepackage[svgnames]{xcolor}
%\usepackage{fontspec}
\usepackage{mathptmx}
\usepackage[nottoc,notlot,notlof]{tocbibind}
\usepackage[numbers]{natbib}

\begin{document}

For example, I'm using $\boldsymbol{x}_t$ and \textit{vector de estados} within a paragraph.
\end{document}

mathptmx is very old and has no bold fonts, a better times clone is newtxmath

\documentclass{book}
\usepackage[spanish]{babel}
\selectlanguage{spanish}
\usepackage[utf8]{inputenc}
\setcounter{tocdepth}{4}
\setcounter{secnumdepth}{4}
\usepackage{parskip}
\usepackage{graphicx}
\usepackage{multirow}
\usepackage{amsmath, calc}
\usepackage[margin=1.5in]{geometry}
\usepackage{hyperref}
%\usepackage{textcomp}
\usepackage{listings}
\usepackage[svgnames]{xcolor}
%\usepackage{fontspec}
%\usepackage{mathptmx}
\usepackage{newtxtext,newtxmath}
\usepackage[nottoc,notlot,notlof]{tocbibind}
\usepackage[numbers]{natbib}

\begin{document}

For example, I'm using $\boldsymbol{x}_t$ and \textit{vector de estados} within a paragraph.
\end{document}