smching,
you were trying to compile an example dedicated to regular arduino: uno, mega...
<SoftSerial> library was not designed only for Digispark, it was also designed for regular arduinos.
"Serial" refers to an hardware UART which do not exist on the Digispark.
As already stressed somewhere in the forum, it's very difficult to manage common libraries for regular arduinos AND for the Digispark.
Regards,