Did you got error something like this, whenever you get update ubuntu "sudo apt-get update" in terminal?
W: GPG error: https://repo.skype.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
W: GPG error: https://repo.skype.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
ANS.
curl https://repo.skype.com/data/SKYPE-GPG-KEY | sudo apt-key add -
in a terminal