Updating from such repository can't be done securely, and is therefore disabled by default
65
What can I do if a repository/PPA does not have a Release file?
118978
Not all updates can be installed: partial upgrade
113379
Updating the repository isn't working
26952
Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'xxx' doesn't support architecture 'i386'
10633
software updater failed to download repository information
36772
Is it possible to only allow specific packages updates from a PPA
157415
Other than via configuration in /etc/yum.repos.d, where else can a repository be configured?
158674
How do I get `apt-get update` to work on a machine with no internet access and manually assigned IP address?
59768
How do I enable the "multiverse" repository?
138044
How can I stop the Software Updater from nagging me to restart?
43425
failure to load repository information
26745
Skipping acquire of configured file 'contrib/binary-i386/Packages' as repository ... doesn't support architecture 'i386'
13107
How to install updates via command line?
66285
Package repository down, how to relocate?
15413
sudo apt update always giving Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
12054
How do I deal with "unauthenticated sources" errors in the Software Center?
154815
How do I add a repository from behind a proxy?