Death Note is infamous for the intellectual conflict between Light and L. With both of them possessing intellect that surpassed an ordinary person, their match was a series of back-and-forth blows that built up until their final confrontation. In this battle of the minds, they often took advantage of whatever assets they could in order to outmaneuver the other. But, as a recent theory posted to Reddit proposed, Light perhaps overlooked his greatest asset, which could have resolved their feud in a matter of a few chapters or episodes.
The theory notes Ryuk’s craving for apples is a sort of flaw to this character. It is stated in the anime that apples are almost like a drug to some shinigami. Considering the fact that the apples in the shinigami realm lack any sort of flavor, these beings are partial human apples, especially. Without a supply of apples, Ryuk is even shown to go through withdrawal symptoms.
Of course, this is not to say that Ryuk can’t get apples on his own. Shinigami are invisible to those who haven’t come into contact with the Death Note or one of its many pages. Thanks to that, it’s quite plausible that Ryuk could steal apples from his surroundings if he wanted to.
The only complication is that Ryuk has an interest in seeing what Light will do with the Death Note and has to accompany him wherever he goes. Therefore, it served as beneficial to both Light and himself to find blind spots in whatever cameras were set up in his room. While Light could use these blind spots for himself if he needed to stay out of sight, Ryuk could use those blind spots to eat apples without being spotted by other humans.
By using Ryuk’s craving for apples as a basis for their argument, the theorist supposes that Light could have possibly manipulated Ryuk into revealing L’s true name in exchange for more apples. And since Light neglected to remember this as he became more and more concerned with defeating his enemies, this is probably one of his biggest mistakes in Death Note.
The biggest flaw in the theory is that the rules of the Death Note categorically don't allow for this situation to occur. This fact is clearly stated in Part 18 of the book’s rules: "The god of death must not tell humans the names or life spans of individuals he/she sees." It would cause Ryuk considerable trouble if he was found to be intentionally breaking the rules.
Now, this is not to say that shinigami can’t find a way around certain rules. The very fact that Light has a Death Note in the first place is that Ryuk “dropped” the Death Note from the shinigami realm, avoiding the restriction of shinigami not being allowed to lend their books to humans. Ryuk claims that he was bored in the other realm, and therefore this serves as a sort of entertainment to him.
The biggest piece of evidence to support the theorists' point is that Light avoids using the deal for the shinigami eyes -- a method in which a shinigami grants Death Note users the eyes of a shinigami in exchange for half of their life span. If Light had these eyes, he could have figured out L’s true name simply by looking at him.
However, doing so would technically fracture the god complex that Light built up for himself while he acted as Kira, which is the crux of the theory's point. While Light could use Ryuk to help in smaller cases, such as identifying if there were any blind spots in his room’s camera coverage, using this outside help to expose L’s identity would mean that Light couldn’t do it without sacrificing part of his life. And Light's own arrogance wouldn't allow him to this anymore that it might allow him to manipulate Ryuk's apple cravings.
Despite how simple it would be for Light to coerce Ryuk into revealing L’s name, the Death Note still forbids it from happening. Bribing Ryuk to contravene a direct rule like that with serious consequences would take an awful lot of apples to pull off.