Display issue prevents opening GUI in Debian 10.

[Darío César Luzuriaga Sorribes]

Hi. I updated my OS to Linux Debian 10 from 9. And I'm having a big issue when 
trying to start any graphical interface. I get the message: "cannot open 
display". I worked out in the OS updates already. The computer isn't new, but 
it was running perfectly with Debian 8 and 9 in the past. Now it looks like 
there is an issue with my VGA display driver or similar. I have an AMD Kabini 
Radeon HD 8400 series display. The Kernel driver in use is Radeon and the 
Kernel modules used are Radeon and Amdpu, according to the command lspci -nnk | 
grep -i -EA3 “3d|display|vga”. There is no hardware issue, since I can run 
Windows 10 on that computer too. Does anybody know  what to do in this case? 
Thanks. Dario.