I just completed a Sitecore Postman API sample setup tutorial. This should help. https://www.youtube.com/watch?v=QOMMXT3Fiy0&t=78s
If you did setup the Postman API samples you should see the "CatalogAPISamples" folder. There are many APIs that you can use.
Also if you set up the "Sitecore.Commerce.Engine.SDK.4.0.102.zip" on your local machine you should be able to debug and see what the problem is.
I have another Sitecore Tutorial that explains how to set up the local environment.
https://www.youtube.com/watch?v=eNWCxoA0JUo