Installing Master on Debian 9 Error

https://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/
drill down to the latest version of the nonfree amd64 iso-cd netinst.iso
burn to cd, then boot from the cd, Graphical Install, English, United States,
American English, domain name, don't enter root password, user name/password,
guided use entire disk, all files in one partition, United States, Mate,
SSH Server, standard system utilities, install GRUB Yes,
Debian desktop environment, Mate, SSH server, standard system utilities

sudo nano /etc/lightdm/lightdm.conf
to log in without your user name a password uncomment
and add your user name
autologin-user=john
autologin-user-timeout=0

Open the Synaptic Package Manager
search for linux-image and install linux-image-latest version-rt

reboot the pc
in a terminal type uname -a it should report back PREEMT RT
Linux mesa-eth 4.9.0-7-rt-amd64 #1 SMP PREEMPT RT Debian 4.9.110-3+deb9u1 (2018-08-03) x86_64 GNU/Linux

sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install dirmngr
sudo apt-key adv --keyserver hkp://keys.gnupg.net --recv-key 3cb9fd148f374fef
sudo apt-get install software-properties-common
sudo add-apt-repository "deb http://buildbot.linuxcnc.org/ stretch master-rtpreempt"
sudo apt-get update
sudo apt-get install linuxcnc-uspace

I get this error:
W: GPG error: http://buildbot.linuxcnc.org stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF1B07FEE0EE663E
W: The repository 'http://buildbot.linuxcnc.org stretch Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.