In case you're running Ubuntu USB live distro and trying to install libqt5websockets5 or libqt5websockets5-dev package and your Ubuntu system cannont find it:
$ sudo apt install libqt5websockets Reading package lists... Done Building dependency tree Reading state information... Done E: Unable to locate package libqt5websockets
In case you are going to install libqt5websockets5 package, system will fail to do it and will show you following error in terminal:
$ sudo apt install libqt5websockets5 Reading package lists... Done Building dependency tree Reading state information... Done Package libqt5websockets5 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 'libqt5websockets5' has no installation candidate
it surely means that you are missing official Ubuntu repositories.
Solution is very simple and easy: add (activate) needed repository: