The requirements being 60% legitimacy and 2 warlords (or Greenpire) integrated, right? You also don't need to be past era 3, can do it any era now.

i did that and still nothing happened

Wait, YOU can't make it happen as the player?
I've never had that issue. I only ever have a problem with the AI never progressing (or at least not in the manner I want them to).

In my last campaign as the New Republic, the empire only advanced when I killed their current leader. Also, the Dark Empire didn't appear until I had killed every Imperial faction except the PA. It was a rather odd. And Thrawn never showed up because I killed Isard and then the GE was promptly integrated into the PA. That campaign was a bit weird.

Who are you playing as? The Dark Empire option only appears for any of the Empire warlord factions (GE, PA, EA, ZE, GM). If you're not playing as any of them, it'll be entirely up to the AI to decide, which it's terrible at doing and may never have the chance to if all 5 of those factions are getting beat up and can never reach the required legitimacy/integration.