Hi Thorsten and welcome to Forum. I am sorry to hear about your experience. Let's solve it together. I have 2 or 3 possible solutions for you.
Even though the requirements specifically mention Amiga Forever 7, 8 or 9 only, it should work with the 2015 version too. The problem is that AmiKit is looking for a mounted ISO, a drive called AmigaForever:
1) The easiest one should be to install Amiga Forever on your Windows and then AmiKit will pick it up automatically.
2) Or you can contact Cloanto so they deliver the AmigaForever installer to you again, and then you can make an ISO out of it thanks to its «Build Image» feature https://www.amigaforever.com/screenshots/build-image/
(providing it was part of Amiga Forever 2015 already).
3) Or, providing you have the content of such ISO stored on your harddrive, you can mount such a directory as a drive in AmiKit and call it AmigaForever. To do so, run AmiKit launcher and click Configure. In WinUAE go to «CD&Hard drives» section and click the «Add Directory button». Then enter the following details:
Device name: CD0
Volume label: AmigaForever
Path: <path to your folder with Amiga Forever files> You can do so by clicking the «Select Directory» button.
Just make sure that the directory you choose really contains the complete Amiga Forever folders like:
If there is a correct ROM in AmiKit/RabbitHole/ROMs folder named kick.rom (including rom.key file too), it should work.
Hope it helps?