In this article we will "trick" Boot Camp Assistant to allow us to create the installation USB stick.
If your Boot Camp looks like the screenshot below (the first option is missing),
you should be able to resolve this issue with this article.
First, we need to go to the Boot Camp Assistant Content folder. In order to do so, go to Finder -> Utilities - > Right-click on Boot Camp Assistant - > Show package content -> Content
Right-click and then click again on Show Package Contents:
In this folder, we will copy the file info.plist and backup it.
After doing a backup, right-click on info.plist - > Open with - > Other
Open the file with TextEdit.
Using Spotlight to search for "System Information."
We need to copy the exact "model idintifier" from this screen.
Back in TextEdit we search for the key "PreUSBBootSupportedModels" (your file could be different from the one in the image).
We modify this key to be "USBBootSupportedModels" (we remove the "Pre").
We add in the "<array>" bellow a "<string>" that will contain our model identifier copied in Step 6. (You can see this in the video at the start of this article at 2:14)
SAVE THE FILE! and then close TextEdit
Replace this new file (if you are working in a backup) with the original in the Boot Camp Assistant Content folder.
In Reboot Boot Camp Assistant, you should see something like this now:
You are done! You should be able now to create the Installation disc.