Some Xperia models (like the Z1 Compact) have been reported to experience bootloader unlock failures in newer Flashtool versions. Version 0.9.18.6 consistently succeeds where others falter.
Even with version 0.9.18.6, users often encounter specific error codes. flashtool 0.9.18.6
If you cannot get version 0.9.18.6 working, consider these options: Some Xperia models (like the Z1 Compact) have