I want to set up a wireguard que to tunnel my traffic at home. so I need a simple client. but all the other additional things cannot be installed.
do you have an idea?
So you want to simply install wireguard ? 
if you can tell more about the error, we maybe can help.
and why do you need kali linux to solve your debian problem ? are you doing pentesting or what ?
I use Kali for years and in my opinion it is the most easy and trouble free distribution you can deal with.
I want to install wire guard but the problem is the other packages you need to install for the using of wire guard cannot be installed with the old stable repository so I have to change the repository again and I am afraid of killing my other qubes again so this is my problem.
I don’t understand what you say.
Clone the template so you have a new “wireguard” template - make the
changes there, and install wireguard if you can.
This is good practice, because it will not “kill your other qubes”. If
it doesn’t work out, just delete the cloned template.
Tip - it’s often useful to try out such configuration changes in a qube
first. The changes wont be persistent, but you can find out what
packages are required or resolve any issues, before committing in to a
template.
sure I did yesterday.
changing thesources.list to testing and installed wireguard now I try to find out which one is the best gui.
I don’t understand what you are not understanding you asked me about what the failure is and I tell you exactly what the problem is.