Are you referring to this character, True? It does look like you should qualify. I think the guild leader being away for 4100+ days, and you being rank one does fit the criteria.
You might try resetting your User Interface, just in case something is interfering. If that doesn’t work, you’ll likely need to submit a ticket and see if we can help.
Keep in mind that our staff is generally hands-off guild management, so be sure to specify that your character is Rank 1 and should qualify for dethrone.