The 21.04 is based on Debian bullseye, which does not have qt5-default package in the repository. I reported a bug to launchpad about missed qt5-default package.

There is a way is to install all dependencies of qt5-defalt package with

sudo apt-get install qtbase5-dev qtchooser qt5-qmake qtbase5-dev-tools

and then try to compile your application.

If you are installing some package from third-party which requires qt5-default package on Ubuntu 21.04 (or newer), then you can create fake package by using commands below:

sudo apt-get update

sudo apt-get install equivs



cd ~/Downloads

cat <<EOF > qt5-default-control

Package: qt5-default

Source: qtbase-opensource-src

Version: 5.99.99+fake-13ubuntu37

Architecture: all

Depends: qtbase5-dev, qtchooser

Suggests: qt5-qmake, qtbase5-dev-tools

Conflicts: qt4-default

Section: libdevel

Priority: optional

Homepage: http://qt-project.org/

Description: Qt 5 development defaults fake package

EOF



equivs-build qt5-default-control

sudo apt-get install ./qt5-default_5.99.99+fake-13ubuntu37_all.deb

and enjoy.

But IMO better way is to install Ubuntu 20.04 LTS instead to live another 4 years without problems as it has needed qt5-default package in place.

This explains that qt5-default became obsolete, and that's why it was removed.