Install Qt 5

I am trying to install QT 5, using this document:

https://wiki.qt.io/Install_Qt_5_on_Ubuntu

Here is the result: 

nov5-6@nov56-desktop:~$ sudo apt-get install qt5-default
[sudo] password for nov5-6: 
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Package qt5-default 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 'qt5-default' has no installation candidate
nov5-6@nov56-desktop:~$ 

I need to install Qt5 because my app is Qt 5 based.

I was hoping to use "maintenance too;" but getting this error :

/home/nov25-1/.cache/qt-unified-linux-online

How / where can I modify this reference ?

PS

this error messge is the last entry in .log file .

I am still loking /asking for source of this messge , if it can be changed.

ADDENDUM -bug ?? related to above

Should   I be able to use ":maintenance tool" "Archive " option to get Qt 5 ?

It posts RED warning  ( security) message and NO OTHER option to actually continue.