Nivlac Circle : race is neither claimable nor attackable without claim. Cannot be conquered? Bug?

Solution 1:

Yup, it's a bug. According to this forum post this is from an event that hasn't completed correctly.

There was, or maybe still is, a bug in the event that forms their nation where they "exist" visually but not in the game. The AI for the Nivlac Circle won't do anything, you can't kill them off and you can't attack them.

According to this bug report, you can fix it by altering your save file:

If you edit the save to add ethics, and change the empire "type=nivlacs" to "type=default" it does appear to fix them in-game. Mostly anyways, as they end up as despicable neutrals in that case.