Hello!

I tried a lot, I googled like 15 hours and I still cannot find a solution because whatever they suggested on other threads/forums, it didn't work for myself. Most of the time, it was command not found and I even found solutions to fix that but it didn't work for me to fix that, too.
Firstly, my problem: I have Windows 10 and wanted to install Kali Sana. Everything worked fine there, my problem is: Either I start by priority in BIOS via Windows Boot Manager, it starts directly Windows 10. Or I decide to boot via SATA1 and it will open the GRUB 2.0 Loader, while it only has as option to start Kali and not Windows 10. Both OS work fine but it bothers me to switch via BIOS every time.
So, one solution is according to Google, to use Boot Repair. I want to do it via the command "add-apt-repository". It says command not found, I find this website for solution this:
http://lifeonubuntu.com/ubuntu-missi...itory-command/

Well, it doesn't work for me, too. When I try "sudo apt-get install software-properties-common", it says cannot allocate/find this package.
I even re-installed Kali again from scratch, it didn't work. I followed this tutorial made by Kali developers themself (I guess) : http://de.docs.kali.org/installation...li-mit-windows
I even tried to manually add menuentry Windows 10 to the GRUB Loader, it did appear as Windows 10 there but it said no Operation System was found, that I have to restart to continue.
I even followed some YouTube videos about how they "fix it". It doesn't work for me, too. And I think that I am not too stupid to follow step-by-step slowly a visual tutorial (video).
Is there anyone that has the same problem and knows how to fix it, specially not that "Standard-solution" that you can find the first few pages of Google, as those didn't work for me. I assume I will have to post some detailed information about my system, perhaps my partitions (GParted) and such. However, I am not sure what to post and specially, how to find out some of it. So I will wait for someone to tell me what to do to solve the problem^^
One thing that might be important as I read my hours through Google, my PC had Windows 8 pre-installed and indeed there are like 9 partitions, while 2 are now for Linux (Kali + GRUB) and one big of 960GB, while the others are a few hundred MB.
I heard that pre-installed OS has usually a small partition including the Windows Loader, however I am not sure how to make use of that to fix my problem.

For such a long text, I have to put it in a nutshell the problem: I dont have Windows 10 detected in GRUB Loader.