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The error message contains:
Failed to fetch updates
This may have been caused by external, manually added software repositories or a corrupted sources file.
details:
W: The repository 'http://ppa.launchpad.net/mpstark/elementary-tweaks-daily/ubuntu xenial Release' does not have a Release file.
W: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
W: See apt-secure(8) manpage for repository creation and user configuration details
As yu can see in http://ppa.launchpad.net/mpstark/elementary-tweaks-daily/ubuntu/dists/ xenial is not supported. You have trusty or utopic version only. But you can use anther repository for Tweaks:
sudo add-apt-repository ppa:philip.scott/elementary-tweaks
sudo apt-get update
sudo apt-get install elementary-tweaks
