ADB device offline on ubuntu, can't unlock the bootloader

sudo ./adb reboot bootloader
error: device unauthorized. Please check the confirmation dialog on your device.

I'm getting the above error while trying to connect my Yureka! to my ubuntu 14.04 LTS Here's a list of what I've tried already and it still doesn't work:-

1. used USB2.0 instead of 3.0

2. added the rule in /etc/udev/rules.d/51-android.rules for my phone

3. adb retstart (kill+start again)

4. Tried deleting the adbkey, but no pop-up comes on my phone's screen while connecting again

5. tried using as a camera instead of MTP

Although, I'm able to transfer files from my phone to laptop and vice-versa, but still unable to connect through adb. I'm trying to flash factory image (LINUX) for my newly bought Yureka as per the steps mentioned here but stuck as I'm unable to unloack the bootlocker

Output of adb devices

List of devices attached 
2a26cfb unauthorized

Following message comes when trying to unlock the bootloader:

Unlocking bootloader
< waiting for device >