Anders from the Shopify Social Care team here. Thanks for reaching out today!
Let me preference my reply here stating that I'm in way proficient with API support but I would like try and help where I can! Have you tried reviewing over our API help docs for common response status codes? Our general API help docs should prove helpful, as well.
That said, do you mind providing a bit more detail in terms of what the API tool you're trying to develop is? Also, any details you can provide about what you're doing up to receiving the error message might prove useful to someone in our developer community. Ideally, someone can better provide you with some advice or insight with just a bit more info.