BootCamp problem - Failed to create partitions on disk

This is what I expected. On a 2017 iMac, we can try to repair this in Single-User Mode, but you will need to post photos of the screen, since there is no other way to capture the screen. Boot and hold Command+S, which will show you a black-and-white console window. At the '#' prompt, run

fsck_apfs -s -o -y /dev/rdisk2

There is a space between each 'word'. It would be typed in as

fsck_apfs<space>-s<space>-o<space>-y<space>/dev/rdisk2

There is another method, if you just want to test Windows. Download VMware Fusion/Parallels/VirtualBox, install W10 in a VM, and test. You do not need to activate W10 for 30 days. You may not see native hardware performance, but it will help you experience Windows on the Mac.

Can you post the output of

sudo fsck_apfs -n -l /dev/rdisk2

This may cause your iMac to slow down.

Type exit at the '#' prompt, which will reboot the iMac in normal mode.

Do you have a Time Machine backup of macOS? If yes, see Restore both macOS and your files section of Restore your Mac from a backup - Apple Support. The 'full' Restore should try to address the APFS corruption.

Since you have a Time Machine backup, what is the possibility of installing Mojave, installing Windows and then upgrading to Catalina and restoring your files (not a full restore) from your Catalina backup?

Can you post the output of

mount -v

Can you run

umount -a

and post the output?

Is BCA still failing to create a partition?