How change written part color in LaTeX block on a tikzposter?

How can I set the written part inside the "block" as white color?

\documentclass{tikzposter}
\usepackage[paperwidth=1080px,paperheight=1980px]{geometry}
\usepackage{xcolor}

%\usetheme{Board}

\begin{document}

% See Section 4.1
\column{0.7}
\block{A test!}{
}


\end{document}

\end{document}

This one is compiling if you run it on overleaf

Solution

Several problems:

tikzposter already loads the geometry package. You must not load it with different options afterwards. An error messages in the log will tell you that!
There is a missing ; after \node .... An error message in the log will tell you that!
\block has two mandatory arguments - you must not use it with only one argument. This will cause an error message!
finally you can change the text colour of the block title with \colorlet{blocktitlefgcolor}{red}
you don't need to load the xcolor package, this is already taken care of by tikz

\documentclass{tikzposter}
\geometry{paperwidth=1080px,paperheight=1980px}

\usetheme{Board}

\begin{document}

\node[above right,opacity=1.2,inner sep=0pt,outer sep=0pt] at (bottomleft) {\includegraphics[width=\paperwidth,height=\paperheight]{example-image-duck}};

\maketitle % See Section 4.1
\colorlet{blockbodybgcolor}{black}
\colorlet{blocktitlefgcolor}{red}
\block{\textbf{Ultrastructural anylisis}}{}
\end{document}