Q: Getting an error with Bootcamp Assistant while making a Windows 10 bootable USB

Create an ISO image for Boot Camp from Windows installation media This is not needed if you already have a .iso file.

Use Windows 10 on your Mac with Boot Camp

How to install Windows using Boot Camp

Boot Camp Help

1. From the Finder, open the Applications folder, and then Utilities.
2. Open the Boot Camp Assistant, and then click Continue. The next screen should give you a list of options.
3. UITS recommends making the USB drive installer first. You don't need to install Windows or download the support software at this time, as it will be easier to do both later. To proceed:
   1. Verify that the USB drive you will be writing to is plugged in.
   2. Uncheck the Install Windows 7 or later version and Download the latest Windows support software from Apple options.
   3. Check Create a Windows 7 or later install disk and click Continue.
4. Your USB drive should be listed in the "Destination disk" area. Use choose to browse to your .iso file; after selecting it, click Continue.
5. If prompted, confirm your action and/or provide an administrator's password. The process of writing the .iso file to the USB drive can take 20 minutes or longer.

Do you have a legitimate copy of the Win10 .iso file? Is the USB flash drive properly partitioned and formatted per the other installation instructions? I don't use Boot Camp or Windows, so I'm not sure why you are having this problem. You appear to be following the instructions I found for creating the installer flash drive.

I'm sorry, then. I don't know what to suggest.

Here's another tutorial: How to Create a Windows 10 Installer USB Drive from Mac OS X

That's a strange one. Glad you have it working.