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