I tested your instruction and it leads to an error message if sudo echo 'deb https://apt.rb24.com/ rpi-stable main' > /etc/apt/sources.list.d/rb24.list
is executed.
Yes, I also found it, but couple of hours after posting.
This fails
pi@raspberrypi:~ $ sudo echo 'deb https://apt.rb24.com/ rpi-stable main' > /etc/apt/sources.list.d/rb24.list
bash: /etc/apt/sources.list.d/rb24.list: Permission denied
It is a permission issue and requires user root.
This works
#First switch from user pi to user root
pi@raspberrypi:~ $ sudo -i
root@raspberrypi:~#
#Now give the command which was failing
root@raspberrypi:~# echo 'deb https://apt.rb24.com/ rpi-stable main' > /etc/apt/sources.list.d/rb24.list
#EXIT user root and back to user pi
#DO NOT stay in root for more than necessary
root@raspberrypi:~# su pi
pi@raspberrypi:/root $ cd
pi@raspberrypi:~ $
Check if the above command has created the file "rb24.list"
pi@raspberrypi:~ $ ls -l /etc/apt/sources.list.d/
total 8
-rw-r--r-- 1 root root 195 Nov 28 21:36 raspi.list
-rw-r--r-- 1 root root 42 Apr 2 09:33 rb24.list
Check it has written line 'deb https:// apt.rb24.com/ rpi-stable main' to the file 'rb24.list'
pi@raspberrypi:~ $ cat /etc/apt/sources.list.d/rb24.list
deb https://apt.rb24.com/ rpi-stable main
Now try update
sudo apt-get update
Hit:1 http://archive.raspberrypi.org/debian stretch InRelease
Hit:2 http://mirrordirector.raspbian.org/raspbian stretch InRelease
Hit:3 https://apt.rb24.com rpi-stable InRelease
Reading package lists... Done