Flashtool-fastboot mode-select kernel to flash

I got a problem when using Flashtool. Could someone help me plz..~

My Sony z3 compact hasn't been bootloader unlocked. I tired to flash the kernel .sin from a stock rom in fastboot mode---
>sending boot
>OKAY
>writing 'boot'. . .
>FAILED (remote: Command not allowed)

Why it wouldn't even let me flash the kernel.sin extracted from a stock firmware:confused:

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