The "Oh Crap!" potty training method strictly advises against using stickers, candy, or any other external rewards [1, 2]. The author views using the toilet as an expected, necessary life skill, much like learning to walk, and points out that you wouldn't give your child M&Ms for taking their first steps [3].
Here is why the method strongly discourages rewards and sticker charts:
What you should use instead of rewards:The ultimate goal is for your child to develop an internal sense of
pride and self-mastery [9]. To encourage them naturally, you can offer verbal praise ("You did it!"), a high-five, or let them do the highly coveted task of carrying their little pot to dump the contents into the big toilet—a process most kids are incredibly proud of and fascinated by [10-12].