$ sudo apt-get install build-essential libreadline-dev libssl-dev libpq5 libpq-dev libreadline5 libsqlite3-dev libpcap-dev git-core autoconf postgresql pgadmin3 curl zlib1g-dev libxml2-dev libxslt1-dev vncviewer libyaml-dev curl zlib1g-dev Reading package lists... Done Building dependency tree Reading state information... Done Package vncviewer is a virtual package provided by: xtightvncviewer 1.3.10-0ubuntu3 tigervnc-viewer 1.7.0+dfsg-7ubuntu1 You should explicitly select one to install. E: Package 'vncviewer' has no installation candidate
All this means is that there is no package that exists in your sources list that matches that name exactly. The best approach (IMHO) to take when faced by an issue like this is to begin by checking Ubuntu Package Search using the keyword of what you were trying to find. In this case "vncviewer" is actually embedded in the valid package name. Even if it wasn't you can still search the package contents for specific filenames which can also be a successful way of locating which package contains a command/program you are attempting to locate. Either of which should get you where you are attempting to go.
In your specific case, you have a couple of easy options. Both of which require enabling the Universe Repository first.
Once that is complete, you can either install the gtk version with
sudo apt install gvncviewer or the X version with
sudo apt install xtightvncviewer
The GTK version has fewer dependencies and a smaller package size. There's a decent (if somewhat dated) Community Help wiki on VNC Clients.