# There's a mistake with this table and I don't find the problem

When I try this table there's a problem at the end of the first row and I don't know how to fix it.

\documentclass{article}
\usepackage[utf8]{inputenc}

\title{A}
\author{AA }
\date{November 2018}

\usepackage{natbib}
\usepackage{graphicx}
\usepackage{tabu}

\begin{document}

\begin{tabu} to 0.9\textwidth { | X[c] | X[c] | X[c] | X[r] | }
\hline
l(cm) & V(V) & I(A) & R(\Omega)\\
\hline
4  & 0,020  & 1,70 & \\
\hline
8  & 0,051  & 1,70 & \\
\hline
12  & 0,077  & 1,70 &8\\
\hline
16  & 0,102  & 1,70 & \\
\hline
20  & 0,127  & 1,70 &\\
\hline
24  & 0,152  & 1,70 &\\
\hline
\end{tabu}

\end{document}

• \Omega will only work in math mode. Jun 9 '19 at 9:33

The tabu package is broken, don't use it. In this case you can just use an ordinary tabular.
Your first error tells you Missing $inserted, because \Omega is a math mode command, you can't use it in text mode. In general, you should make a clear distinction in your code what is math and what is not. For example, the variables in your table head (l, V, I and R) should all be math. For the numbers and units in your table (and anywhere else in your document), I recommend using the siunitx package. From your use of , as a decimal marker I guess that you are writing in German, so I chose that setting in the MWE below. (It is conveniently irrelevant if you use , or . in the input to siunitx macros, though. If you don't get the numbers with , from some other program, I would always prefer to type . in the code since that's compatible with pretty much any program you may want those numbers to interact with.) For tables, I recommend using the booktabs package. Have a look at its documentation and maybe this post for why that's a good idea. Now, here's how I would typeset your table: \documentclass{article} \usepackage{booktabs} \usepackage[locale=DE]{siunitx} \begin{document} \begin{tabular}{S[table-format=2] S[table-format=1.3] S[table-format=1.2] S[table-format=1]}\toprule {$$\displaystyle\frac{l}{\si{\centi\meter}}$$} & {$$\displaystyle\frac{V}{\si{\volt}}$$} & {$$\displaystyle\frac{I}{\si{\ampere}}$$} & {$$\displaystyle\frac{R}{\si{\ohm}}$$} \\\midrule 4 & 0.020 & 1.70 & \\ 8 & 0.051 & 1.70 & \\ 12 & 0.077 & 1.70 & 8 \\ 16 & 0.102 & 1.70 & \\ 20 & 0.127 & 1.70 & \\ 24 & 0.152 & 1.70 & \\\bottomrule \end{tabular} \end{document}  If you don't like the fraction notation in the table head, you can of course use something like {$$l$$ in \si{\centi\meter}} or {$$l$$ [\si{\centi\meter}]} instead. In tables with a lot of columns the fraction notation can be advantageous due to its slenderness, though. (As has been pointed out in a comment by schtandard) \Omega -- like any other greek letter macro -- needs to be run in math mode. In this case you want it to be "inline", so just wrap it in single$, like $\Omega$
 l(cm) & V(V) & I(A) & R($\Omega$)\\