HTC Desire 510 how to remove user code and no data lose ?

Hi guys.
I need help how to remove user lock with digits (not pattern) without data lose.

In moment mobiel status is:
LOCKED with S-FF

When I try to write TWRP I get this error:

Code:

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C:\ADB>fastboot flash recovery twrp.img
sending 'recovery' (12432 KB)... OKAY
writing 'recovery'... FAIL not allowed
FAILED (remote: not allowed)
Execution time is 1(s)

C:\ADB>

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I read that problem is with locked bootloader.

How to unlock it without data lose ?

p.s. I want to delete files locksettings.db, locksettings.db-shm, locksettings.db-wal, password.key andgesture.key