Hello,
I wondered what was this 'nodm' that i saw while attempting to install some softwares (not only Qt)
Thank you for the advices JMCC, this was indeed the problem. As the Os i used was a Bionic which i had configured with 'armbian-config' so that to have a desktop (but perhaps incorrectly), i follow your advice and i flashed Debian Strech. Indeed, this Os has a desktop environment by default, and for me everything works nicely (wifi, network); eveything i needed could be configured without using command lines. Then i installed Synaptic and i used it to install QtCreator, QtDesigner, Qt4.8.7, ... and also CodeBlocks; everything was installed without problem and works.
I feel that i found the good Os, even if Bionic is perhaps a little nicer; perhaps i will retry with Bionic later, but from my little experience with linux on the Rock64, i can say that Armbian with Debian Strech is the more easy to install and setup.
thanks again JMCC for your judicious advices.
Regards