public key is not available

Get:18 http://mirror.ppa.trinitydesktop.org/trinity/trinity-r14.0.0/debian stretch InRelease [12.2 kB] Get:19 http://mirror.ppa.trinitydesktop.org/trinity/trinity-builddeps-r14.0.0/debi… stretch InRelease [12.0 kB] Err:18 http://mirror.ppa.trinitydesktop.org/trinity/trinity-r14.0.0/debian stretch InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C93AF1698685AD8B Err:19 http://mirror.ppa.trinitydesktop.org/trinity/trinity-builddeps-r14.0.0/debi… stretch InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C93AF1698685AD8B ... E: The repository 'http://mirror.ppa.trinitydesktop.org/trinity/trinity-builddeps-r14.0.0/debian stretch InRelease' is no longer signed.