W: The repository 'http://ppa.launchpad.net/t-tujikawa/ppa/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/t-tujikawa/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
This PPA does not support your version of ubuntu. You can see that from visiting the URL http://ppa.launchpad.net/t-tujikawa/ppa/ubuntu/dists/. There is not much you can do except hoping that it will be updated (not likely as this repository seems abandoned for a long time) or find another PPA for your purpose.