I think we would like to see exactly what command you wrote and the error, its great if you can find a way to cut&paste so we avoid "raspbian" / "raspbain" errors.
raspbain raspbian on my pi.
Yes but which version ? did you upgrade to Buster from Stretch or just installed a fresh image ?
But it probably means that Debian just released Buster: https://www.debian.org/News/2019/20190706
I think it might be good enough to replace "testing" with "stable" in the apt-source files, but it also might be safest to wait a little bit to see if there is some official guide how to deal with this.