Try this:
# Import key to apt-key
apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 1D043681
# Move old source
sudo rm -f /etc/apt/sources.list.d/rb24.list
# Create a new debian repository source file
sudo echo 'deb https://apt.rb24.com/ rpi-stable main' > /etc/apt/sources.list.d/rb24.list
# Update apt
sudo apt-get update
.