https://packages.debian.org/jessie/qt5-default
It is available , you shouldn't be having any problems.
running apt install qt5-default:
You are not very clear on exactly how you are trying to install it.
"apt install anypackage" would not work anyway.
If you have not updated your apt resources lately , that may be the problem.
Try running
first.
read the apt-get manual,..
update
update is used to resynchronize the package index files from their
sources. The indexes of available packages are fetched from the
location(s) specified in /etc/apt/sources.list. For example, when
using a Debian archive, this command retrieves and scans the
Packages.gz files, so that information about new and updated
packages is available. An update should always be performed before
an upgrade or dist-upgrade. Please be aware that the overall
progress meter will be incorrect as the size of the package files
cannot be known in advance.
Then in the part on upgrading it says :
An update must be performed first
so that apt-get knows that new versions of packages are available.
I do not use apt-get that much, usually I use synaptic, there is a option to try to fix or repair broken packages, which is what you need to do before you can continue.
apt-get has the same options, looking at the manual myself now, it appears to have the same options.
-f, --fix-broken
Fix; attempt to correct a system with broken dependencies in
place. This option, when used with install/remove, can omit any
packages to permit APT to deduce a likely solution. If packages
are specified, these have to completely correct the problem. The
option is sometimes necessary when running APT for the first time;
APT itself does not allow broken package dependencies to exist on
a system. It is possible that a system's dependency structure can
be so corrupt as to require manual intervention (which usually
means using dselect(1) or dpkg --remove to eliminate some of the
offending packages). Use of this option together with -m may
produce an error in some situations. Configuration Item:
APT::Get::Fix-Broken.
Lots if details here :
https://www.debian.org/doc/manuals/debi ... 02.en.html