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      Public key missing Apt

      Open the terminal (Ctrl+Alt+T) and run the following command:

      sudo apt-get update

      Now wait till it finishes and see if you see something like this:

      W: GPG error: http:/something.something Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXXXXXXX

      Note down the numbers you see after NO_PUBKEY. This error is because the application’s public key has not been added. To solve the error use the following command in the terminal:

      sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com XXXXXXXXXXXXXXXX

      After adding the missing public key run the following command (again):

      sudo apt-get update