Formating slide

you have many issues with your slide:

• in beamer table environment is not a float, so it not need any positioning option (in your case H, which is ignored)
• referencing tables without having captions has no sense
• if you use \caption,referencing has sense if captions are numbered
• use \multicolumns or each cell in the second table is superfluous
• in second table you define 5 columns but use only three ...
• I doubt that column headers in the second table have math variables, correct is use \textit{Antes} etc
• for vertical align of your tables you can add option [t] to minipages or use tabularx as I do in the following MWE
• I also suggest to use S columns type in the second table

Considering aforementioned i suggest the following rewriting of your slide:

\documentclass[aspectratio=169]{beamer}
\usetheme{Warsaw}
\setbeamertemplate{headline}{}
%----
    \setbeamertemplate{caption}[numbered]
    \setbeamerfont{caption}{size=\scriptsize}
%----

\usepackage{siunitx}
\usepackage{tabularx}

\begin{document}
        % SLIDE 40
        \subsection{Sistema de 33 barras}
\begin{frame}
    \frametitle{Resultados}
    \framesubtitle{Sistema de 33 barras (barras candidatas: 7, 8, 9 e 10) - Primeiro caso}
\begin{table}
\begin{tabularx}{\linewidth}{@{} *{2}{>{\centering\arraybackslash}X} @{}}
    \caption{Resultado com $w_1 = 0$, $w_2 = 0$ e $w_3 = 1$:}
    \label{primeiro_caso_trintaetres_barras}
\begin{tabular}{|c|c|c|}
    \hline
Filtro  & Ordem Harmônica   & Barra \\ \hline
1       & 5ª                & 7     \\ \hline
2       & 3ª                & 10    \\ \hline
\end{tabular}
&
    \caption{Perdas totais de potências ativas e reativas:}
    \label{primeiro_caso_dezessete_barras}
\begin{tabular}{|>{$}l<{$}|S|S|}
    \hline
    & {\textit{Antes}}  & {\textit{Depois}} \\ 
    \hline
\Delta P_{\mathrm{total}} (kW)
    &   1385,291        &   1353,611        \\ 
    \hline
\Delta Q_{\mathrm{total}} (kVAr)
    &   395,617         &   387,007         \\ 
    \hline
\end{tabular}
\end{tabularx}
    \end{table}
\end{frame}
\end{document}

The right-hand tabular environment is defined to contain 5 columns, but it has only 3.; do get rid of the final two. Moreover, the right-hand tabular environment contains a lot of very clumsy and cluttery code which simply isn't needed. You should simplify it drastically, maybe along the lines shown in the code below.

\documentclass[aspectratio=169]{beamer}
\usetheme{Warsaw}
\setbeamertemplate{headline}{}
\usepackage[portuguese]{babel}
\usepackage{array,siunitx}
\sisetup{output-decimal-marker={\text{,}}}
\begin{document}
% SLIDE 40
\subsection{Sistema de 33 barras}
\begin{frame}
\frametitle{Resultados}
\framesubtitle{Sistema de 33 barras (barras candidatas: 7, 8, 9 e 10) -- Primeiro caso}

\setlength\extrarowheight{2pt} % for a more open "look"

\begin{minipage}[t]{.475\textwidth}
\centering
Resultado com $w_1 = 0$, $w_2 = 0$ e $w_3 = 1$:

\begin{tabular}{|c|c|c|}
\hline
Filtro & Ordem Harmônica & Barra  \\ \hline
1      & 5ª              & 7      \\ \hline
2      & 3ª              & 10     \\ \hline
\end{tabular}
\end{minipage}\hfill
\begin{minipage}[t]{.505\textwidth}
\centering
Perdas totais de potências ativas e reativas:

\begin{tabular}{|l|S[table-format=4.3]|S[table-format=4.3]|}
\hline
& {\textit{Antes}} & {\textit{Depois}} \\ 
\hline
$\Delta P_{\!total}$ (kW) & 1385,291 & 1353,611 \\ 
\hline
$\Delta Q_{total}$ (kvar) &  395,617 &  387,007 \\ 
\hline
\end{tabular}
\end{minipage}

\end{frame}
\end{document}