Get:25 http://ppa.launchpad.net/levi-armstrong/qt-libraries-xenial/ubuntu xenial/main arm64 Packages [912 B]
Get:26 http://ppa.launchpad.net/levi-armstrong/qt-libraries-xenial/ubuntu xenial/main Translation-en [3,340 B]
Fetched 22.3 kB in 2s (8,978 B/s)
Reading package lists... Done

ubuntu@robotsp1:/etc/apt/sources.list.d$ sudo apt-get install qt57creator-plugin-ros
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package qt57creator-plugin-ros


the sudo apt update shows that all sources have been found and no errors

Hit:15 http://ports.ubuntu.com/ubuntu-ports xenial-security InRelease
Hit:16 http://ppa.launchpad.net/levi-armstrong/qt-libraries-xenial/ubuntu xenial InRelease
Hit:17 http://ports.ubuntu.com/ubuntu-ports xenial-updates InRelease
Hit:18 http://packages.ros.org/ros/ubuntu xenial InRelease
ubuntu@robotsp1:/etc/apt/sources.list.d$ sudo apt-get install qt57creator-plugin-ros
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package qt57creator-plugin-ros