Disk Utility Can't Mount Volume. Error -69502

AC6VV-is-me wrote:

Thanks to errors I made when I erased and reloaded everything in the iMac, there is now an unused, undesired 1TB partition in the "container" along with the operating system. The successful installation is in "iMac HD" and "iMac HD - DATA". The lost space is in the old "Macintosh HD" partition, because I removed its companion volume. Disk Utility us unable to mount the offending volume, so there is no way to address the problem and remove it entirely.

https://discussions.apple.com/content/attachment/07b07970-ed3d-4fa5-b548-f5ff9bffb03e

Yes you have borked the installation—

From Interent Recovery/bootable USB installer/TM bootable BU >>Disk Utility>View>Show All Devices.

Here you need to erase/reformat/initialize the on the parent Drive level not the Container level or child Volume level.

Reinstall the macOS, and restore your user data from a backup:

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904