Estou recebendo o seguinte erro ao tentar instalar o Skype
Failed to download repository information
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E:The repository 'http://archive.ubuntu.com/ubuntu yakkety Release' does not have a Release file.
W:Updating from such a repository can't be done securely, and is therefore disabled by default.
W:See apt-secure(8) manpage for repository creation and user configuration details.
E:The repository 'http://security.ubuntu.com/ubuntu yakkety-security Release' does no longer have a Release file.
W:Updating from such a repository can't be done securely, and is therefore disabled by default.
W:See apt-secure(8) manpage for repository creation and user configuration details.
E:The repository 'http://in.archive.ubuntu.com/ubuntu yakkety Release' does no longer have a Release file.
W:Updating from such a repository can't be done securely, and is therefore disabled by default.
W:See apt-secure(8) manpage for repository creation and user configuration details.
E:The repository 'http://in.archive.ubuntu.com/ubuntu yakkety-backports Release' does no longer have a Release file.
W:Updating from such a repository can't be done securely, and is therefore disabled by default.
W:See apt-secure(8) manpage for repository creation and user configuration details.
E:The repository 'http://in.archive.ubuntu.com/ubuntu yakkety-updates Release' does no longer have a Release file.
Você está usando o Ubuntu Yakkety Yak (16.10) que está fora do suporte e EOL (End of Life).
Você precisa atualizar para uma versão mais recente do Ubuntu. Como o Ubuntu 16.10 irá atualizar para o 17.04, que também é EOL, você precisa atualizar para 17.10.
Você pode fazer isso executando sudo do-release-upgrade .