errorslyxsymbols
I'm trying to export a PDF of a LyX document (it has never caused a problem before). Below I have posted pictures of the error message along with the equation LyX highlighted for me. I don't know what \hbox is, and I didn't use that in my code. Can someone please help me edit the equation so that I can export a PDF?
As egreg mentions in a comment, the reason you get the error is that neither the amsfonts nor amssymb package, where \mathbb is defined, is loaded. To force LyX to add one of these you can do one of two things:
Go to Document --> Settings --> LaTeX preamble and add there either
\usepackage{amssymb}
or
\usepackage{amsfonts}
Go to Document --> Settings --> Math options. Uncheck Use AMS math package automatically and check Use AMS math package, as in the screenshot below. LyX will now have added
\usepackage{amsmath}
\usepackage{amssymb}
to the document preamble.
The first of these, the automatic one, adds these packages only if you use some of their features through the LyX GUI. E.g. if you do Insert --> Math --> AMS Align, the amsmath package will be added, and if you e.g. add some text with Blackboard bold via the math font button on the toolbar -- 
-- then amssymb will be added.
See also an answer concerning the use of the package vmargin
% arara: pdflatex
\documentclass[a4paper,11pt]{article}
\usepackage{vmargin} % needed for the command \setmarginsrb (I would recommend geometry here)
\usepackage{graphicx}
\usepackage{array,multirow,makecell} % removed second call of array
\usepackage{fix-cm}
\usepackage[utf8]{inputenc}
\usepackage{indentfirst}
\usepackage{amsmath} % I recommend mathtools here (not needed in your example)
\usepackage{eso-pic} % loads xcolor
\usepackage{fancyhdr}
%\usepackage{color} % xcolor is newer
\usepackage{hyperref} % in most cases to be loaded last
\pagestyle{fancyplain} \chead{}\lhead{\textit{The Creative Workers}} \rhead{\emph{\textit{FPSDesigner}}}
\pagenumbering{arabic}
% Margins
\renewcommand{\baselinestretch}{1.5}
\setmarginsrb{4.5cm}{2.5cm}{3.5cm}{2.5cm}{0pt}{0pt}{0pt}{0pt}
\begin{document}
\thispagestyle{empty}
\vspace*{2cm}
\begin{center}
test
\end{center}
\end{document}
Best Answer
The command
\coloneqqis unknown, you need to load a package for this, such asmathtools,txfontsorpxfonts.Add to your document preamble:
LyX 2.1 will have direct
mathtoolssupport.