I have looked at the other threads with this topic but not getting anywhere.
I have an unpublished, public app that has been working fine on a demo store.
Now, I have successfully authenticated and installed it on a production store. It is making graphql API calls and reading data just fine, but I am getting the error when I try to POST. The post works perfectly from Postman with the same X-Shopify-Access-Token, so I have no idea why it's not working when sent from the app. Clearly the token is valid. Again, everything works fine on the demo store as well.
What's making the calls? (your tech stack). Using postman shows the call is right but more likely something is incorrect or misconfigured in your stack.
Have you been able to log exactly what your app is sending? Ideally with headers sent, and response returned. Looking at the request and response tend to reveal a few clues into why you're seeing the error.