Curly brackets don't show.

Try using them like this:

$\left\{\right\}$

You have to use them in mathmode and put a backslash in front of the bracket.

when you compile the following example you will get the error

Missing $ inserted \left

The example:

\documentclass[]{scrreprt}
\begin{document}
\left\{blah blah \right\}
\end{document}

You see the error is clear.

\documentclass[]{scrreprt}
\usepackage{amsmath}
\begin{document}
$\left\{\mbox{blah blah}\right\}$ %without amsmath

$\left\{\text{blah blah}\right\}$ %with amsmath
\end{document}

Please create a minimal example. It is easier for use.

\documentclass[]{scrreprt}
\usepackage{amsmath}
\begin{document}

\{ $\floor{\frac{3}{3}}$ + $\floor{\frac{3}{3}}$ + $3$ \} + 
\{$\floor{\frac{3}{3}}$ + $3$ + $0$ \} = $9$. % example with amsmath

\end{document}