NO_PUBKEY Error after installing SSL


I’m new to this, followed guidance on setting up ssl on my apache server, hosted on debian 9 based vps, hosted on GCloud.
I was successful on setting up the ssl, however, I get the following NO_PUBKEY error after running an update:
sudo apt-get update.
Here’s my result.

Hit:1 stretch/updates InRelease
Hit:2 cloud-sdk-stretch InRelease                                              
Hit:3 google-compute-engine-stretch-stable InRelease                           
Ign:4 stretch InRelease                                                                      
Hit:5 stretch-updates InRelease                                                              
Hit:6 google-cloud-packages-archive-keyring-stretch InRelease                        
Hit:7 stretch-backports InRelease                                                            
Hit:8 stretch Release                                                  
Get:9 disco InRelease [21.3 kB]                     
Hit:10 stretch InRelease
Ign:9 disco InRelease
Fetched 21.3 kB in 0s (27.6 kB/s)
Reading package lists... Done
W: GPG error: disco InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C47BE8E75BCA694
W: The repository ' disco 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.

You used the wrong installation instructions.

Remove the PPA and follow


Thank you. This worked just perfectly.
Much appreciated.