It seems you want to format the numbers using siunitx, while knitr also tries to do that automatically. Only one of them should be used, so either you do not use \num{}, or tell knitr to output the numbers without scientific notations (this is perhaps what you prefer), e.g. \Sexpr{as.character(max(degree))}, or use other functions like format(), sprintf(), and so on, to turn the numbers to character strings, so that knitr will no longer do scientific notations.
(Re-wrote the answer after the OP changed the table in the MWE.)
The following solution lets you use the S column type for the four "GWh" columns and lets you use a tabularx environment (to assure that the width of the table is equal to \linewidth). The trick -- such as it is -- consists of using S for the numbers and C (a centered version of X) for the headers.
You'll observe that I've reorganized the table's header. Your original setup requires line-breaks for all four important header words -- Producción, Consumo, Exportaciones, and Importaciones. I think it's better to avoid (as much as possible) the hyphenation of such words. I left the square brackets around the GWh headers; however, they may not be needed.
(To simplify and streamline the preamble code, I've also removed all packages that don't appear to be essential to generating the table.)
\documentclass[fontsize=10pt,paper=letter,headings=small,bibliography=totoc,
DIV=9,headsepline=true,titlepage=on]{scrartcl}
\usepackage[spanish,mexico]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tabularx,booktabs}
\newcolumntype{C}{>{\centering\arraybackslash}X} % centered version of "X" column type
\newcommand\mc[1]{\multicolumn{2}{@{}C@{}}{#1}} % shortcut macro
\usepackage{siunitx} % Paquete para insertar unidades
\sisetup{
per-mode = symbol,
output-decimal-marker = {.},
group-minimum-digits = 4,
range-units = brackets,
list-final-separator = { \translate{and} },
list-pair-separator = { \translate{and} },
range-phrase = { \translate{to (numerical range)} },
}
\ExplSyntaxOn
\providetranslation [ to = Spanish ]
{ to~(numerical~range) } { a } % substitute the right word here
\ExplSyntaxOff
\begin{document}
\begin{table}
\caption{Mercado de energía eléctrica en Norteamérica}
\label{tab:emna}
\begin{tabularx}{\linewidth}{@{} l
*{2}{S[table-format=7.0]r}
S[table-format=5.0]r
S[table-format=5.1]r @{}}
\toprule
País & \mc{Producción} & \mc{Consumo} & \mc{Exportaciones} & \mc{Importaciones} \\
\cmidrule(lr){2-3} \cmidrule(lr){4-5} \cmidrule(lr){6-7} \cmidrule(l){8-9}
& [\si{\giga\watt\hour}] & Fecha & [\si{\giga\watt\hour}] & Fecha
& [\si{\giga\watt\hour}] & Fecha & [\si{\giga\watt\hour}] & Fecha \\
\midrule
Canadá & 612000 & 2007 & 530000 & 2006 & 50120 & 2007 & 19660 & 2007 \\
Estados Unidos & 4167000 & 2007 & 3892000 & 2007 & 20140 & 2007 & 51400 & 2007 \\
México & 243300 & 2007 & 202000 & 2007 & 1278 & 2007 & 482.2 & 2007 \\
\bottomrule
\end{tabularx}
\end{table}
\end{document}
Addendum: Here's the same table, but without the reorganization of the header material. The code is the same as above, except that a Y column type is used for four of the header cells.
....
\newcolumntype{Y}{>{\hspace{0pt}\RaggedRight\arraybackslash}X} % allow hyphenation
....
\begin{table}[htbp]
\setlength\tabcolsep{4pt}
\caption{Mercado de energía eléctrica en Norteamérica}
\label{tab:emna}
\begin{tabularx}{\linewidth}{@{}l
*{2}{S[table-format=7.0]r}
S[table-format=5.0]r
S[table-format=5.1]r @{}}
\toprule
País
& \multicolumn{1}{Y}{Producción [\si{\giga\watt\hour}]} & Fecha
& \multicolumn{1}{Y}{Consumo [\si{\giga\watt\hour}]} & Fecha
& \multicolumn{1}{Y}{Exportaciones [\si{\giga\watt\hour}]} & Fecha
& \multicolumn{1}{Y}{Importaciones [\si{\giga\watt\hour}]} & Fecha \\
\midrule
....
Best Answer
I found a solution... I guess the
command needs a final placeholder to know where it is in the sequence of degrees, arcminutes, and arcseconds.
The solution to my problem is