Raspian Buster install not working

Hello, i tried to install snapd on Raspi4 buster but the package will not be found . maybe there must be an extention in the sources.list which is not described in the install steps?

well, snapd is in the standard debian buster archives:

https://packages.debian.org/buster/snapd

since raspbian is a re-build of debian i dont think there is any reason for it to not also be in the raspbian standard archives.
i don’t think you should need to add anything to your sources.list.