Welcome to the forum scoed. When you click on the cooking pot, scroll down and you should see 'Phantom Gruel' listed. If it is still grayed out, then you are missing an ingredient. When you highlight the recipe it will tell you what you need.
I know I have done this quest a few times and have not had any problems. If anything, save your game and completely exit out of the Sims Medieval. Then start your game up again to see if that helps.