In The Strange Case of Dr. Jekyll and Mr. Hyde, why did the protagonist commit suicide if he could have made a potion?
I would refer you back to the last chapter of this excellent novel to re-read it very carefully. If you look at the last letter of Dr. Jekyll and the explanation of what happened to him and how he started transforming into Mr. Hyde, you will see that what brought him to a crisis point was the way that he started transforming into Mr. Hyde without the potion, obviously symbolising the way that his evil nature was growing stronger and taking control of his good side. When the original salt that was used to make the potion ran out, Dr. Jekyll tried to obtain more, but note what happened:
I sent out for a fresh supply, and mixed the draught; the ebullition followed, and the first change of colour, not the second; I drank it and it was without efficiency. You will learn from Poole how I have had London ransacked; it was in vain; and I am now persuaded that my first supply was impure, and that it was that unknown impurity which lent efficacy to the draught.
Thus Dr. Jekyll found himself not being able to make any more potion due to the "unknown impurity" of the first batch of salt. This leaves him trapped in a terrible position, with the evil part of him growing stronger every hour, and with suicide as the only escape.