i'm pretty sure voting for blockland to be greenlit wouldn't allow you to /greenlight
It was a promotional thing where if you click the link in the greenlight GUI on the main menu, you could use /greenlight to have a green light.
OT: Is this on your server or someone else?
Cause if light is disabled for example in a minigame, you can't use /greenlight.
Or maybe the host has it disabled through a mod overwriting the greenlight server command.
I doubt that blockland knows if you actually voted for it on the link.
If this isn't the cause, try deleting that line from prefs.cs and restarting blockland and clicking the link again.