DarkSite option is available under LCM → setting . 

You have loged in using different user, can you please login using admin user. and try again.

It seems I find the solution. 

Using configure_lcm --enable_darksite -u xxxxx could change the mode to dark site. And I could do inventory sacn now.

The problem is that I could not see the setting. All blank. 

Checked the log, it seems always checking from Internet, but our env could not visit the internet.

2023-11-21 06:21:52,288Z ERROR 78076080 download_utils.py:753 Failed to get file size of file at 'https://download.nutanix.com/lcm/2.0/master_manifest.tgz.v2.sign' with cmd 'c'curl', '-s', '-S', '-f', '-L', '-I', u'https://download.nutanix.com/lcm/2.0/master_manifest.tgz.v2.sign']'.
RV: 7
 Stdout:
Stderr: curl: (7) Failed connect to download.nutanix.com:443; Operation now in progress

So I guess system is keep trying and no feedback.
 

if it is stuck at the load issue i suggest to reach nutanix support