参见 apt-secure(8)

Question

当我输入时sudo apt update我得到了这个：

Hit:1 http://mirror.0-1.cloud/ubuntu focal InRelease
Hit:2 http://mirror.0-1.cloud/ubuntu focal-updates InRelease                                                             
Hit:3 http://mirror.0-1.cloud/ubuntu focal-security InRelease                                                            
Hit:4 http://ppa.launchpad.net/go-for-it-team/go-for-it-stable/ubuntu focal InRelease                                    
Hit:6 http://ppa.launchpad.net/linuxuprising/apps/ubuntu focal InRelease                                                 
Hit:7 http://packages.microsoft.com/repos/code stable InRelease                                                          
Hit:8 http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu focal InRelease                                            
Ign:9 https://repo.protonvpn.com/debian stable InRelease                                                                 
Hit:10 https://brave-browser-apt-release.s3.brave.com stable InRelease                                                   
Hit:11 http://ppa.launchpad.net/teejee2008/timeshift/ubuntu focal InRelease                                              
Hit:12 https://download.sublimetext.com apt/stable/ InRelease                                                            
Err:13 https://repo.protonvpn.com/debian stable Release                                                                  
  Could not handshake: Error in the pull function. [IP: 104.26.8.21 443]
Hit:14 http://ppa.launchpad.net/xuzhen666/gnome-mpv/ubuntu focal InRelease                                               
Get:5 http://repo.mysql.com/apt/ubuntu focal InRelease [5,309 B]
Err:5 http://repo.mysql.com/apt/ubuntu focal InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Reading package lists... Done        
E: The repository 'https://repo.protonvpn.com/debian stable Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://repo.mysql.com/apt/ubuntu/dists/focal/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: The repository 'http://repo.mysql.com/apt/ubuntu focal InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

我该如何解决这个问题？任何帮助都将不胜感激。