I accidentally got similar errors starting YaST Partitioner or YaST Bootloader in a VM with test disks, one of which contained broken (btrfs) filesystem. I do not think btrfs is relevant here, what is relevant is the fact that yast failed to mount it. In my case I could continue normally and information related to the other devices was correct. I assume in case of OP because error is related to the device with root filesystem it lead to the final error (no device mounted on /).
So in OP case it indicates some problems with the root filesystem. One should troubleshoot it in usual way - checking logs, trying the same commands that failed in CLI, observing their output etc.