what's wrong? I don't understand these lines (bootloader) slot-count: not found (bootloader) slot-suffixes: not found (bootloader) slot-suffixes: not found (bootloader) has-slot

artition: not found
Code:
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:partition: not found
Code:
fastboot flash partition gpt.bin
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:partition: not found
target reported max download size of 536870912 bytes
sending 'partition' (45 KB)...
OKAY [ 0.013s]
writing 'partition'...
(bootloader) Validating 'gpt.default.xml'
(bootloader) Image primary_gpt failed validation
(bootloader) Preflash validation failed
(bootloader) Cancelling 'gpt.default.xml'
FAILED (remote failure)
finished. total time: 0.122s
They are normal
The line that causing your error is
(bootloader) Image primary_gpt failed validation
(bootloader) Preflash validation failed
Reasons why are in my previous post but it's because your device has a locked bootloader and is looking for a specific signature from the firmware you are flashing.
To me I would guess the device was previously unlocked as it states system modified but you have relocked the bootloader as it states oem-locked
If this is the case the chances are you will never get gpt & bootloader to validate especially if the re-locking process wasn't done correctly
You can try different firmware as stated in my previous post or take to a repair shop