Cannot install TWRP on Nexus 6 (Android 7 installed) via MacOS

Question

Purpose:

Install TWRP into Nexus 6 with Android 7 via macOS. (I have tested in Windows, it works)

Problem:

I have a nexus 6 device with Android 7 installed. I would like to install TWRP into Android 7, but I cannot copy TWRP to my nexus. The error message is the followings,

  $fastboot flash recovery recovery.img
  (bootloader) has-slot:recovery: not found
  target reported max download size of 536870912 bytes
  sending 'recovery' (0 KB)...
  FAILED (remote failure)
  finished. total time: 0.000s

May I know how do I solve the issue?

Environment and Steps

macos EI Caption, Nexus 6 (Android 7), twrp-3.0.2-0-shamu.img

I performed the following commands,

$adb reboot bootloader
$fastboot flash recovery recovery.img 
// error here. 
// recovery.img which is twrp-3.0.2-0-shamu.img, just rename it. 

I have googled, and someone said it's caused by locked OEM devices. however, my device is unlocked.