I think you want to run this on the unraid device such as /dev/md# so you're doing it on the drives inside the parity protected array.
If you run it on /dev/sdx#/ then you are invalidating the parity since you are writing below the devicemapper. Also, you need to run it on a partition, such as /dev/sdc1 (first partition). Specifying /dev/sdc is for the entire drive. That is why it can't find any valid filesystems..
The /dev/md# syntax may have changed on 6.12 unraid versions for ZFS tomfoolery, so check what the right device name is. I'm still on 6.11 where it's /dev/md1 for first partition of the first drive in the array.
Only use /dev/sdX1 if you are recovering to another drive.