Unable to unlock bootloader for HTC 816G android phone
I only have experience with an old HTC Wildfire S so please be patient with me here...

• The first step towards unlocking Any HTC  bootloader is to make an official account with HTC Dev.    HTCDev.com      

( Become a HTC Developer -  so to speak )

• Secondly you need to Unlock the bootloader using the official HTC Bootloader unlock tools... 
 HTC Dev.com/bootloader   

The process usually goes as follows...


Enter Fastboot  
Enter Command fastboot oem get-unlock-token


You place the unlock token into the website and then it will convert the Token into a Token.bin or something like that ( unlock.bin ) i can't remember, i said bare with me... 


You place the (.bin) file next to your fastboot.exe program ( not ADB ) and then run   
fastboot oem unlock -pathToUnlock.bin  


You will get appropriate instructions on the HTC Dev site so don't worry about my crappy guide ;) ... 

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P.S
ADB is only uses while the device is "Live" .. Fastboot is used to flash things like the System...

ADB is a part of the system, Fastboot is part of the Boot... Different programs for different things.