I Can't Finish My Cake

@EllieSimmmmmer Looks like the reason your Sim can't finish their cake is because the hired chef/caterer is using the oven. In your screenshots the hired chef is in the process of cooking a cake, which means the oven is being used by them for part of that cake cooking process and so unable for any other Sim to use. Dismiss the chef then your Sim should be able to complete cooking their cake, or place a second oven.