synaptic manager error 16.04

in synaptic manager some softwares are marked all upgrades, when i try to Reload i get an error message saying

The repository 'http://ppa.launchpad.net/claudiocn/slm/ubuntu xenial Release' does not have a Release file.Data from such a repository can't be authenticated and is therefore potentially dangerous to use.See apt-secure(8) manpage for repository creation and user configuration details.Failed to fetch http://ppa.launchpad.net/claudiocn/slm/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not FoundFailed to fetch http://ppa.launchpad.net/claudiocn/slm/ubuntu/dists/xenial/main/binary-i386/Packages 404 Not FoundSome index files failed to download. They have been ignored, or old ones used instead.