If you are trying to fine-tune GPT-3.5 or GPT-4o-mini, based on the documentation, you need to create a JSON Lines file (.jsonl) with all the samples for training.
I created a simple project to validate each JSON Lines object, so you can make sure each sample has a valid structure for training.
Link on Github: github . com/nuveraxgroup/openai-fine-tune-gpt-schema