Is this a Shopify app? Either way, be sure to have the proper scopes inside your custom app or root .toml file.
If you changed the scopes during the process uninstall and reinstall to get access via the oAuth process.
I know it sounds silly to mention, but be sure you are using the correct method ("GET" | "POST"), and proper data structure for the body/query params (if at all). I mess up these small things all the time. After batting my head against the wall I realize its a small type or something as simple as this, haha.
Any other details will help better answer though. Be sure to like and reply so I know to respond. Good luck, hope this helps a bit.