The situation is a bit tricky because even if it won't take up any space, the `tikzpicture` will still have to start a line and `tcolorbox` you use afterwards will go in a fresh line.
### Approach 1:
You can works around this by placing your `tikzpicture` into the theorem box:
```
\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\setbeamersize{text margin left=10mm,text margin right=5mm}
\setbeamertemplate{frametitle}[default][center]
\usepackage[many]{tcolorbox}
\usepackage{listings}
\tcbuselibrary{skins}
\tcbset{
% arc=0pt,
% outer arc=0pt,
colback=white,
}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
shapes,
tikzmark, shapes.geometric}
\usetikzmarklibrary{listings}
\newtcolorbox{mytheorem}[2][]{%
text width=7.5cm, text height=1.5cm, enhanced,colback=white,colframe=framecolour, coltitle=comcolor,
sharp corners,boxrule=0.8mm,
attach boxed title to top left={yshift=-0.3\baselineskip-0.4pt,xshift=2mm},
boxed title style={tile,size=minimal,left=0.5mm,right=0.5mm,
colback=white,before upper=\strut},
title=#2,#1
}
\definecolor{comcolor}{RGB}{10,161,119}
\definecolor{framecolour}{HTML}{009980}
\begin{document}
\begin{frame}[t]
\frametitle{Summary}
\begin{mytheorem}[colframe=red, text width=10cm]{}
\begin{tikzpicture}[remember picture,overlay, scale=0.9, transform shape]
\node[draw, overlay, very thick, rounded corners, fill=green!30, anchor=north west, xshift=12cm, yshift=-0.5cm] at (current page.north west) {hello};
\end{tikzpicture}test
\end{mytheorem}
\pause
\begin{enumerate}[<+->]
\setlength\itemsep{5pt plus 1fill}
\item one
\item two
\item three
\item four
\item five
\item six
\end{enumerate}
\end{frame}
\end{document}
```
### Approach 2:
you could also try to hide the space from the additional line:
```
\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\setbeamersize{text margin left=10mm,text margin right=5mm}
\setbeamertemplate{frametitle}[default][center]
\usepackage[many]{tcolorbox}
\usepackage{listings}
\tcbuselibrary{skins}
\tcbset{
% arc=0pt,
% outer arc=0pt,
colback=white,
}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
shapes,
tikzmark, shapes.geometric}
\usetikzmarklibrary{listings}
\newtcolorbox{mytheorem}[2][]{%
text width=7.5cm, text height=1.5cm, enhanced,colback=white,colframe=framecolour, coltitle=comcolor,
sharp corners,boxrule=0.8mm,
attach boxed title to top left={yshift=-0.3\baselineskip-0.4pt,xshift=2mm},
boxed title style={tile,size=minimal,left=0.5mm,right=0.5mm,
colback=white,before upper=\strut},
title=#2,#1
}
\definecolor{comcolor}{RGB}{10,161,119}
\definecolor{framecolour}{HTML}{009980}
\begin{document}
\begin{frame}[t]
\frametitle{Summary}
{\nointerlineskip%
\begin{tikzpicture}[remember picture,overlay, scale=0.9, transform shape]
\node[draw, overlay, very thick, rounded corners, fill=green!30, anchor=north west, xshift=12cm, yshift=-0.5cm] at (current page.north west) {hello};
\end{tikzpicture}%
}%
\begin{mytheorem}[colframe=red, text width=10cm]{}
\end{mytheorem}
\pause
\begin{enumerate}[<+->]
\setlength\itemsep{5pt plus 1fill}
\item one
\item two
\item three
\item four
\item five
\item six
\end{enumerate}
\end{frame}
\end{document}
```
### Approach 3 (preferred):
Personally, I would not use the crude `\pause` command and use normal overlays instead. This has the advantage, that you can place the `tikzpicture` at the end of the top aligned frame, so nothing bad should happen even if it might add one more line
```
\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\setbeamersize{text margin left=10mm,text margin right=5mm}
\setbeamertemplate{frametitle}[default][center]
\usepackage[many]{tcolorbox}
\usepackage{listings}
\tcbuselibrary{skins}
\tcbset{
% arc=0pt,
% outer arc=0pt,
colback=white,
}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
shapes,
tikzmark, shapes.geometric}
\usetikzmarklibrary{listings}
\newtcolorbox{mytheorem}[2][]{%
text width=7.5cm, text height=1.5cm, enhanced,colback=white,colframe=framecolour, coltitle=comcolor,
sharp corners,boxrule=0.8mm,
attach boxed title to top left={yshift=-0.3\baselineskip-0.4pt,xshift=2mm},
boxed title style={tile,size=minimal,left=0.5mm,right=0.5mm,
colback=white,before upper=\strut},
title=#2,#1
}
\definecolor{comcolor}{RGB}{10,161,119}
\definecolor{framecolour}{HTML}{009980}
\begin{document}
\begin{frame}[t]
\frametitle{Summary}
\begin{mytheorem}[colframe=red, text width=10cm]{}
test
\end{mytheorem}
\begin{uncoverenv}<+->
\begin{enumerate}[<+->]
\setlength\itemsep{5pt plus 1fill}
\item one
\item two
\item three
\item four
\item five
\item six
\end{enumerate}
\end{uncoverenv}
\begin{tikzpicture}[remember picture,overlay, scale=0.9, transform shape]
\node[draw, overlay, very thick, rounded corners, fill=green!30, anchor=north west, xshift=12cm, yshift=-0.5cm] at (current page.north west) {hello};
\end{tikzpicture}%
\end{frame}
\end{document}
```
