It's also not the Tree of Knowledge, it's the Tree of Knowledge of Good and Evil. And that presents a problem:
If Adam and Eve did not yet understand what is and is not a good thing to do, they could not possibly have understood that it was not good to disobey God. Eve did not know the serpent was evil. And yet he punishes Adam and Eve for doing what they did not realize was wrong of them to do.
My money's on it being a pomegranate originally. Apples wouldn't have existed in the fertile crescent over 2000 years ago. Pomegranates are also messy and look bloody when eating them, fitting the "carnal knowledge" side of the story. I've heard other people suggest they could have been dates, but pomegranates seem like a way better fit for the story.
Etymology
The word apple, whose Old English ancestor is æppel, is descended from the Proto-Germanic noun *aplaz, descended in turn from Proto-Indo-European *h₂ébōl.[3]
As late as the 17th century, the word also functioned as a generic term for all fruit, including nuts. This can be compared to the 14th-century Middle English expression appel of paradis, meaning a banana.[4]
So yes... We have no idea what the fruit actually was. Because all fruit were basically called "apple".
Apple is probably the most common interpretation because a lot of languages use it as kind of a vague fruit term, and the Bible has been retranslated and reinterpreted roughly one million times. The French call potatoes apples
It's been depicted as various things in old art and literature. Apple is very common. But you also find figs, grapes, pomegranates, and occasionally pears. Probably some others I missed.
Imo it's much more likely to have never been an actual piece of fruit. The story is probably an allegoric warning for some taboo subject and now we can only speculate on what was originally meant.
My theory: Adam and Eve were convinced by Adam's one eyed snake to fornicate, but then their dad found out and kicked them out.