Captured Soul of Arachnoxia

When I hover over the map it says that Arachnoxia is there and it's soul can be captured. When I put the vessel and the map in the device it says that the boss of that map's soul cannot be captured.

https://pathofexile.gamepedia.com/Captured_Soul_of_Arachnoxia just says you need the map and the vessel to capture the soul, but that's clearly not the case. What do I have to do to capture this soul for the Pantheon upgrade? I have googled and watched a couple of videos and they all say the same as the wiki, just put the map and vessel in the device and you're good to go.

I have the exact same issue, and everywhere i look, nothing seems to have a legit answer as how to solve it.