On https://www.fusetools.com/docs/basics/installation-and-quickstart your code to copy and paste has two errors:
<App Background="#some nice color">
<ClientPanel>
<StackPanel ItemSpacing="10">
<Text FontSize="30">Hello, world!</Text>
<Slider />
<Button Text="Button" />
<Switch Alignment="Left" />
</StackPanel>
<ClientPanel>
</App>
#some nice color is obviously not valid, and also the closing ClientPanel tag is missing a ‘/’.