Can not install Win 10 (Bootcamp 6.1)

I did a fresh osx install and wanted to install window 10 back but bootcamp is asking for a usb drive, high sierra wasn't asking it, win10 iso file was enough. Even I try to install with fat formatted usb drive, I still can not install. (Note: I have a default fusion disk came with iMac but not using it. Installed 500gb ssd.)

Video: https://youtu.be/aVuB6MHjnzk

See the screenshots:

Trying without USB connected . I wasn't asked for a usb when I was on high sierra in same computer. Want to install to 500gb ssd(macOS container) :

Trying with a fat formatted USB disk:

Also tried the way without bootcamp as described here: https://fgimian.github.io/blog/2016/03/12/installing-windows-10-on-a-mac-without-bootcamp/

and again no luck :

Before you ask, here is my diskutil list:

diskutil list                                                                                21:33:22
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         499.9 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +499.9 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume macos                   41.5 GB    disk1s1
   2:                APFS Volume Preboot                 44.3 MB    disk1s2
   3:                APFS Volume Recovery                517.0 MB   disk1s3
   4:                APFS Volume VM                      2.1 GB     disk1s4

/dev/disk2 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         28.0 GB    disk2
   1:                        EFI EFI                     314.6 MB   disk2s1
   2:                  Apple_HFS apple ssd               27.6 GB    disk2s2

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *30.8 GB    disk3
   1:                 DOS_FAT_32 WININSTALL              30.8 GB    disk3s1

I don't want to downgrade to high sierra. What should I do?

I did NVRAM, PRAM, and SMC reset