Yes, there might be. The Naruto and Attack on Titan fandoms are both huge, and fans often like to cross over their favorite characters in fanfiction. So it's quite possible to find Eren - from Attack on Titan - in Naruto fanfiction. However, it may not be as common as fanfictions within each respective universe.
The relationship in 'Attack on Titan Eren x Reader Lemon Fanfiction' is a creative take on the characters. Since it's fanfiction, the writer has the freedom to shape the relationship in various ways. Eren's character traits, like his determination and strong will, can be intertwined with the reader's character (even if it's a self - insert or a more general 'reader' character). The 'lemon' tag often indicates a more adult - themed relationship exploration, which could include romantic encounters, but it doesn't have to be just about that. It could also be about the emotional closeness that develops between them in the face of the challenges in the 'Attack on Titan' world. For example, they might find solace in each other during the chaos of the titan attacks, and this shared experience could deepen their bond in ways that are unique to the fanfiction's narrative.
I'm not sure if there is such a specific fanfic. Fanfics cover a wide range of themes and scenarios, but this one seems rather unique. It might exist in some niche corners of the fanfic community. You could try searching on dedicated fanfic websites using relevant keywords to see if you can find it.
Fanfiction about Eren and Levi kissing can be quite interesting. It might explore a different side of their relationship. Some fanfictions in this regard could be about a moment of emotional intensity between them, where the kiss represents a deep connection that goes beyond their usual interactions in the original story.