Authentication problem with my PPA
Solution 1:
Most likely this is caused by a missing PGP key in your APT keyring. You can add the key with the following command:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 72D340A3
Of course you need to replace the key with your PPA key.
Also, the following command will add the key automatically:
sudo add-apt-repository ppa:user/ppa-name
After adding the key, you need to run apt-get update to download and verify the signature.