Well, I mean, it appears langchain just technically doesn't support structured response for Ollama (according to the link above). But, as I've said, I have absolutely no idea what all this middle-layer stuff actually does and what may be the reason why different vendors have different integration capabilities in this regard.
I'm totally new (and maybe somewhat late) to the domain, literally just tried right now to automate a fairly simple task (extracting/guessing book author + title in nice uniform format from a badly abbreviated/transliterated and incomplete filename) using plain ollama HTTP-API (with llama3 as a model), but didn't have much success with that (it tries to chat with me in its responses, instead of strictly following my instructions). I think, my prompts must be the problem, and I hoped to try the langchain, since it somehow seems to abstract the problem, but saw that it isn't supported for a workflow the OP used. But since this is a field where I'm really totally new, I suppose I also may be making some more general mistake, like using a model that cannot be used for this task at all. How would I know, they all look the same to me…
Ollama project itself is fairly stingy with explanations. Doubtfully there are many people out there trying to automate an answer to the "Why is the sky blue?" question.
So, I wonder, maybe somebody knows a more digestible tutorial somewhere, explaining this stuff from the ground up?