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/debian 
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/debian 
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.