Access to requested resource is denied

I created the IAM users, policy and roles based on this link:

https://developer-docs.amazon.com/sp-api/docs/creating-and-configuring-iam-policies-and-entities

Then I successfully registered my application and was trying to use them in Postman to get the data from SP API, based on this link:

https://developer-docs.amazon.com/sp-api/docs/using-postman-for-selling-partner-api-models However, in Postman, it keeps on saying "Access to requested resource is denied" and the error code is 403. I checked online and the reason might be something related to the users access, but I don't know how to resolve it.