However, I didn’t find any info on the “Getting started” or any other place about how to solve this problem,…
Without changing nothing, it just allowed me to join all the traces… I don’t understand 
Thanks for your time
Glad you solved the problem. One common cause for this problem is that you might have accidentally drawn the PCB traces (or parts of them) using the line tool and not using the trace tool. While (decorative) lines can be drawn on traces or anywhere else (kicad doesn’t do DRC for decorative items), you can’t draw traces on top of with lines.
If there is nothing you didn’t change, you must have changed everything 
Seems unlikely. Graphic lines do not change into wires without user interaction. some kind of other user error seems more likely. I don’t consider it “solved”, but there is no use in thinking too deep about either way. Just move on, build some experience with KiCad and over time you’ll learn to either avoid or analyze and fix a myriad of small things without even thinking much about them. Maybe it was just one of those monday morning things.
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No I mean they probably just have literally drawn graphic lines (without changing them into wires). It’s not easy to tell the difference, they both just look like red* lines (and the missing net name of the lines might not be obvious).
* depending on the layer of course.
I am using linux and I have always problems with staff, my friends use it without any trouble… but not me… 
THANKS EVERYONE, I learnt a lot
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