Missing $ inserted in a table

Question

I'm getting this error:

! Missing $ inserted.
<inserted text> 
                $
l.557                            & V_
                                     n \le 1 kV & 1 kV \le V_n \le 69 kV & 6...

? ! Missing $ inserted.
<inserted text> 
                $
l.557                            & V_n \le 1 kV &
                                                  1 kV \le V_n \le 69 kV & 6...

? 

And this is my table, can anyone help me where should i put the $? Latex is already doing it by itself (i guess), but i really want to know whats wrong here.

\begin{table}[h!]
\caption{Valores de referência para as distorções harmônicas totais de tensão.}
\begin{center}
\label{limites_distorcao}
\begin{tabular}{|c|c|c|c|}
\hline
\multirow{2}{*}{Indicador} & \multicolumn{3}{c|}{Tensão Nominal do Barramento}                                                          \\ \cline{2-4} 
                           & V_n \le 1 kV & 1 kV \le V_n \le 69 kV & 69 kV \le V_n \le 230 kV \\ \hline
$DHTvI_i$                   & 7,5\%                  & 6\%                                 & 4\%                                         \\ \hline
$DHTv3_i$                   & 6,5\%                  & 5\%                                 & 3\%                                         \\ \hline
\end{tabular}
\end{center}
\end{table}

Answer 1

You are missing the dollar signs for the math code V_n and \le which causes the error (as already stated by other users in the comments). Inside a tabular every cell is an own box and you need to enable math mode for each one.

In addition to that you have some other issues in your code:

• Don't use the center environment but the \centering command. The former adds unwanted vertical space and is for text paragraphs only. See Should I use center or centering for figures and tables? for further details.
• For the physical units and percent values I recommend the great siunitx package
• Try to avoid vertical rules in your table and try to use the booktab package. Less rules in the table make it look better.
• The use of {table}[h!] is one of the most frequent beginner issue as they don't understand the use of floats (In short: they should float!). If you want a table at a certain position in the text then the table environment is not a good choice (yes, confusing!). See Force figure placement in text and maybe also How to influence the position of float environments like figure and table in LaTeX? for more information.

Here a version of your code with the improvements added:

\documentclass{article}
\usepackage{blindtext}% for example text here only
\usepackage{multirow}
\usepackage{caption} % proper table caption distance
\usepackage{booktabs} % book quality table lines
\usepackage{siunitx} % format SI units
\sisetup{output-decimal-marker = {,}} % use comma as decimal marker

\begin{document}
\blindtext

\begin{table}[h!]
\caption{Valores de referência para as distorções harmônicas totais de tensão.}
\centering % don't use {center} environment, which is for text paragraphs only and add unwanted vertical space before and after
\label{limites_distorcao}
\begin{tabular}{cccc}
\toprule
\multirow{2}{*}{Indicador} & \multicolumn{3}{c}{Tensão Nominal do Barramento}                                                          \\[.25ex]  
                           & $V_n \le \SI{1}{\kilo\volt} $ & $ \SI{1}{\kilo\volt} \le V_n \le \SI{69}{\kilo\volt} $ & $ \SI{69}{\kilo\volt} \le V_n \le \SI{230}{\kilo\volt} $\\ \midrule
$DHTvI_i$                  & \SI{7,5}{\%}                  & \SI{6}{\%}                                 & \SI{4}{\%}                                         \\ 
$DHTv3_i$                  & \SI{6,5}{\%}                  & \SI{5}{\%}                                 & \SI{3}{\%}                                         \\ 
\bottomrule
\end{tabular}

\end{table}

\blindtext

\end{document}