You haven't said which release you're trying to install, but if you check release notes (eg. Lubuntu 20.04.3) you'll find a Known Issues section
Replace Partition on Certain Partition Schemes With Calamares Fails
Installs replacing a partition may fail with this bug. As a workaround; reboot, use KDE Partition Manager to delete the ‘unknown’ partition that exists, create the partition using the format you want (ext4, btrfs & xfs were tested & worked), deactivate swap partitions & exit KDE Partition Manager. Calamares can then use the prepared partition.
You'll note that bug applies to "sfdisk --force --append /dev/sda" which is the message you've posted.
In discussion of the impish release notes this issue came up, and I've written the following which maybe helpful - https://discourse.lubuntu.me/t/calamares-rare-failure-to-create-partition-mkfs-errors/2774 . Summary steps are :-
- Reboot your live session.
- Delete & Re-create your system partition (using KDE Partition Manager)
- Repeat the install but keep your / partition (prepared in step 2)