Boot Camp installation failed.The USB drive is not mounted. Imac 2015 MacOS 11.7

I am trying to load windows 10 on my mid 2015 iMac. I want to use my iMac as a second monitor for my PC laptop.

I have an internal HD (macOS 11.7.6) and external SSD (SanDisk extreme 1TB as Startup Disk (500gb) and data disk (500gb) (macOS 11.7.3)

System "Startup Disk" shows external SSD as "Startup Disk" disk, but "about this Mac" shows internal HD as "Startup Disk".

I launch bootcamp assistant.

I get to the first window (Select Task) with three check marks. I have tried the option with and without the first box checked (USB flash drive installer). According to the instructions i have a "newer" iMac and dont need the USB installer but I created one aways as the 1st method without it didnt work.

At Select Tasks, I leave the 1st box unchecked, check marks on the other two.

At "Save Windows Support Software" screen it only shows the SanDisk external SSD Data Disk as the Destination disk.

When I click continue I get an error.

Boot Camp installation failed.

The USB drive is not mounted.

System Info from "about"

macOS Big Sur

Version 11.7.6

iMac (Retina 5K, 27-inch, Mid 2015)

Processor; 3.3 GHz Quad-Core Intel Core i5

Memory: 24 GB 1600 MHz DDR3

Startup Disk: Internal HD

Graphics: AMD Radeon R9 M290 2 GB