Consider a thought experiment involving 'something' and three individuals attempting to understand it: one person claims it is a red ball, another asserts it is a simulation, and the third insists it is a cat. Now, an omniscient entity enters and declares it to be X. If we define omniscience as possessing complete and certain knowledge (implying infallible justification), it would imply that X is the true nature of the 'something,' rendering the other possibilities false. (Note: A possibility where something could be subjectively or simultaneously be a red ball, cat, 'X' etc. itself is another possibility but it is still in contrast with the other possibilities mentioned before/not same as the before)
However, the omniscient entity cannot disprove the other possibilities as false since the 'something' could potentially be a red ball, a simulation, or a cat, because they are possible explanations or definitions for that something. Basically, if something has R1, R2, R3, etc. ways to exist/explain/occur, then you cannot prove it to be 'X' because that would imply R1, R2, R3 in the first place are not possible ways at all but by definition they are. The only way one could be certain is if somehow 'X' is the only possible way to explain or exist. Even if the entity were infallible, knowing everything without error or deception, they would need to prove this infallibility. Yet, they cannot establish that their knowledge is exhaustive or devoid of falsehood.
Consider another scenario: a being capable of creating a simulated reality with conscious entities. These entities experience subjective qualia, which are unobservable by anyone other than the experiencing entity. This implies that even the creator of the simulation cannot have absolute certainty about the subjective experiences within it. Applying this reasoning to omniscience, if qualia exists (not saying it does for sure, just seeing the implications down this path), there are aspects of reality unknowable to any entity, including an omniscient one. It is conceptually possible for the being to experience that same qualia, but the problem here is, it cannot be certain if it is actually facing the same qualia.
Furthermore, even if we assume a higher power created the universe, it cannot prove its own act of creation, as it would rely on its own perception and could question its memory as being fallible. In conclusion, complete knowledge of reality is impossible for any entity, human or divine, due to limitations in perception and the existence of subjective experiences that elude direct observation or measurement. Claims of omniscience lack a solid foundation and cannot be substantiated. The arguments surrounding omniscience often lead to circular reasoning or infinite regress, and can lead to rendering the concept self-contradictory. The argument is based on the definition of 'omniscience' stated earlier in this question itself, and not on any other definition.