Narrative Coherence refers to the ideal result in the construction of the story: making sure everything fits together, basically.

All stories are fundamentally built out of small individual cause/effect blocks that need to be fitted together to produce a whole. In order for that whole to fit, those blocks need to…y’know, fit each-other. I feel this concept is pretty self-explanatory, so let’s move onto…

Narrative Incoherence is what happens when the narrative isn’t coherent. Essentially, at some point, the writer screwed up – or, gods forbid, did it intentionally – and the narrative came out incoherent. This usually happens when an aspect of the plot or worldbuilding undermines the rest of the plot or worldbuilding, though it can happen in multiple ways.

What I think is important to note (as I’m the one who made these terms up) is that calling a narrative ‘incoherent’ should be reserved for stories that fall apart in a narrative sense. So a story that has a mostly sensible plot bar broken worldbuilding only has broken worldbuilding. But a narrative where the events of the plot and resolutions to mysteries actively undermine, de-legitimize or disqualify the rest of the plot is ‘incoherent’.