It is a known bug in 16.04 that is in the process of being fixed and a new version of Ubuntu Software Centre will be released in your update channel in the next week.
Workaround in the meantime
Open a terminal and cd to the directory where the Manager.deb file is. Then type the following:
sudo dpkg -i Manager.deb
sudo apt-get -f install
When everything is finished you will find Manager installed and everything will work. If Manager had disappeared after an upgrade to Ubuntu 16.04, don’t worry you will not have lost any data