gpg error

Hi, as the title suggests, I'm currently getting the following error... Err:14 https://mirror.apheleia-it.ch/repos/Kolab:/16/Ubuntu_24.04_Plesk_17 InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C8B3DA4DF6491D3D Fetched 1,037 kB in...