You were wrong because the phrase "a unifying or dominant idea in a work of fiction" does not accurately describe the nature of a conflict in a narrative. Instead, conflict refers to an opposing force that creates tension and propels the plot forward.
The correct phrase is "a fight, battle, struggle, or strife," as it encapsulates the essence of conflict—where characters face obstacles or opposition that challenges their goals or desires.
Example from a Disney movie:
In "The Lion King," one of the central conflicts arises between Simba and his uncle Scar. Scar's desire for power leads him to betray his brother Mufasa and create strife within the Pride Lands. Simba struggles with his identity and sense of responsibility after Scar takes control. This conflict drives the plot, as Simba ultimately must confront Scar and reclaim his rightful place as king, depicting a classic struggle between good and evil.