Boot Camp - issues with installing Windows 10 on 2017 iMac

When using Boot Camp Assistant on my iMac, partition gets created and Windows support files downloaded - when it reboots to install Windows I get "Windows cannot locate the disk and partition specified in the unattended answer file’s <ImageInstall> setting. Make sure the setting references a valid partition and restart the installation".

• I have downloaded the Windows 10 IOS from the Windows website
• reset NVRAM
• reset SMC
• no external drives connected
• Time Machine not on and there are no local snapshots
• FireVault is turned off
• Tried this multiple times in Catalina - did not work
• Upgraded to Big Sur - tried multiple times - did not work
• Tried installing with bootable USB, able to get a step further and system asks where I want to install Windows. I select the Bootcamp partition, then receive error "We couldn't create a new partition or locate an existing one. For more information see Setup log files." When I return to Mac OS - it deletes the OSXRESERVE partition.

The Boot Camp Assistant does not give me any options other than following the process. Partition the drive, download software then restart to install Windows.

I have been trying to install this for two days and I don't know what else I can try. Each time I try this I have to reset the hard drive, re-create the partition it, re-download the files. Any other suggestions.