Cannot flash Moto G3 2015 due to "Preflash validation failed" error

I have a Moto G3 XT1550 phone. I had to reset the phone as its UI was restarting on its own after 1-2 minutes of usage. After the phone booted, it asked me to enter the GMail and password the phone was registered with which I did, but the phone failed to recognize the Google account credentials. **I NEVER ACTIVATED USB DEBUGGING FROM DEVELOPER OPTIONS ON 'THIS' DEVICE AS I NEVER NEEDED TO.** I performed "*Wipe data/factory reset"* and "*Wipe cache partition*" from **Recovery Mode** expecting that the stored Google account would be removed. But it did not. Then I came to know that it is called "Factory Reset Protection (FRP)". Since then, I have tried everything I could find over the Internet - Bypassing FRP, flash Stock ROM (100% sure downloaded a correct one), flash custom ROM, and flash TWRP. All to no use. Even tried unlocking the bootloader (got the unlock code from Motorola's website the same). Even that did not work, saying:

C:\MotoG3>fastboot oem unlock XCYJ4AZH542XAQWBJYFF
...
(bootloader) Check 'Allow OEM Unlock' in Developer Options. FAILED (remote failure)
finished. total time: 0.016s

While flashing any image from fastboot, it keeps throwing a **Preflash validation failed** error.

C:\MotoG3>fastboot flash partition gpt.bin
target reported max download size of 268435456 bytes
sending 'partition' (32 KB)...
OKAY [  0.018s]
writing 'partition'...
(bootloader) Preflash validation failed
FAILED (remote failure)

---

C:\Moto G3\Mfastboot>fastboot flash recovery twrp-osprey-2.8.7-test1.img
target reported max download size of 268435456 bytes
sending 'recovery' (11170 KB)...
OKAY [  0.400s]
writing 'recovery'...
(bootloader) Preflash validation failed
FAILED (remote failure)

I've even tried an article that tells a trick to entering the phone's settings by long-pressing a word in the Privacy Policy of the phone and opening "Define". Long pressing on a word just makes the phone vibrate and not select it. Even attaching an OTG drive with the hope to copy FRPBypass APK to the phone's memory work as nothing shows up on connecting the OTG drive (as against expected File Explorer opening up). In a nutshell, I have kind-of soft-bricked my phone. Left with no other option. I don't even expect the Service Center guys to do anything that I haven't tried (expect to change the ROM chip, if ever possible). Any help, please?