I searched for and ran the following command.
sudo apt-get update; sudo apt-get --purge --reinstall install software-center software-properties-common software-properties-gtk
But I still don't have software center. I got this error...
Reading package lists... Done E: Failed to fetch "//mirrors.sonic.net/ubuntu/dists/zesty-updates/universe/source/Sources" 404 Not Found E: Failed to fetch "//mirrors.sonic.net/ubuntu/dists/zesty-backports/main/source/Sources" 404 Not Found E: Failed to fetch "//mirrors.sonic.net/ubuntu/dists/zesty-security/restricted/source/Sources" 404 Not Found E: Some index files failed to download. They have been ignored, or old ones used instead. Reading package lists... Done Building dependency tree Reading state information... Done Package software-center is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source E: Package 'software-center' has no installation candidate