I assume you are already using the correct link with the API key & password provided in the admin panel.
The only other thing you need to make sure is deleting the cookies stored in Postman for the shop domain. If you created an API key, deleted it and created a new app then the new API key doesn't work with the stored cookies.
I had this cookie problem with Postman which caused me to write a custom script when I first started testing the APIs. Later I found out that I had cookies saved under the domain name in Postman (thanks to this post I guess: https://community.shopify.com/c/Shopify-APIs-SDKs/SHOPIFY-API-POST-requests-do-not-work-with-POSTMAN...). So I was pretty sure it is same problem.
Just curious: Is it only the POST attempt which is failing but all the GET attempts work?
Hi @igreisman ,
Check out this post and see if the solution in there helps you. This is a topic that comes up frequently.