Okay so I have the 3" essence summoner plug, but nothing shows up in any of the mixing stations
Confirm the following:
1- You are in one of your unlocked apartments (Nem'yh or the Goblin bog)
2- You are clicking on the top right tab of the crafting menu for the key items crafting.
If its not appearing, or the tab is not appearing, please send me a screenshot of the crafting screen you see, alternatively you can send me a link to your save file. I want to figure what happened out so you can unlock it.
EDIT: I found something, If your safe is old, and you completed the Essence caverns before the Slimette patch, this could have caused the issue, let me know if this is the case.
I have replayed from a clean safe a couple of times and the problem does not seem to appear, my guess is that in-between updates and your save file something might have untriggered the condition required to have the crafting option, this could happen in rare cases depending on when your save was created and what changes have been applied since. I have implemented a redundancy trigger upon defeating the Summoner slime again after the mission. This should allow you to fix the save file check by fighting the boss again. This change should be up and ready on the next patch. The game is pretty much still on Alpha and stuff like this could happen between updates, thank you for reporting it. In the meantime, if you wish to try out what the key unlocks before the next patch, you can start a new save. On the bottom right of the world map there's a blue tree that allows you to access the cheat room. There you can make up for a lot of lost progress if needed. If this is a specific version bug, and the problem persists even at a new save, do let me know, I'll try to cook an early patch for you.