If a cell is producing lactic acid, what molecule is most likely unavailable to that cell

That would be a lactic acid molecule.

Lactic acid, also known as 2-hydroxypropanoic or milk acid, is a compound formed when glucose is broken down under certain conditions in a living creature or by some types of bacteria. In a person, for example, it is an important part of producing energy for strenuous exercise and helps with certain liver functions. During extremely intense exercise, it can buildup to excess and cause short-term burning sensations in muscles. This acid can also be found in certain dairy products, such as yogurt, as well as sourdough breads and some beers and wines as a result of fermentation.