Yes. When I run "fastboot devices" a code appears. When I run the command line with the code (copy pasted from the 4/4 Sony page) i get the same error message - failed
C:\android-sdk\platform-tools>fastboot devices CB521DX3LM fastboot C:\android-sdk\platform-tools>fastboot -i 0x0fce oem unlock 0x12345613624CE9D4