How to Flash Kernel on XPERIA U?

Hello there.

After some days trying, I am not able to flash a kernel. I am facing a simple problem:

Code:

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23/032/2015 11:32:39 - INFO  - Flashing selected kernel
23/032/2015 11:32:40 - INFO  - sending 'boot' (367655 KB)...
23/032/2015 11:32:40 - INFO  - FAILED (remote: The Device must be rooted first)
23/032/2015 11:32:40 - INFO  - finished. total time: 0.002s
23/032/2015 11:32:40 - INFO  - FASTBOOT Output:
 sending 'boot' (367655 KB)...
FAILED (remote: The Device must be rooted first)
finished. total time: 0.002s

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I already have UNLOCKED the bootloader, I don´t understand what I have to do exactly after that, Time Ago I did it right but now i am lost and facing this problem

Thanks for your time