Use the bolded apt-key command to add the GPG public key.
W: GPG error: https://repo.skype.com/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3 W: The repository 'https://repo.skype.com/deb stable InRelease' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details. root@computer:~# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1F3045A5DF7587C3 Executing: /tmp/tmp.UA1ZzOZ6hE/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 1F3045A5DF7587C3 gpg: requesting key DF7587C3 from hkp server keyserver.ubuntu.com gpg: key DF7587C3: public key "Skype Linux Client Repository " imported gpg: Total number processed: 1 gpg: imported: 1 (RSA: 1)