During previous Ubuntu releases, one could easily install Qt 5 by using:
sudo apt install qt5-default
qt5-default package is no longer included with Ubuntu 21.04.
How can I install the Qt 5 development package on Ubuntu 21.04?
qt5-default package is missed in ubuntu 21.04 repository, so you should just install all base packages by yourself by running command below
sudo apt-get install qtbase5-dev qtchooser qt5-qmake qtbase5-dev-tools