Please also include e.g. the endpoint and how you’re authenticating - provide enough detail so that we know everything about what you’re doing (except of course private content or actual authentication keys)
For example: maybe you’re trying to create a topic, or maybe edit one? Not sure.
First of all read through Discourse REST API Documentation, then you can browse the endpoint documentation for the task you’re trying to accomplish; for instance, here is how to create a new topic.