How will I use the SP API?

Hello.
I want to develop integration with amazon sp api.
I’m trying to implement the document on Amazon’s own https://developer-docs.amazon.com/sp-api/docs/what-is-the-selling-partner-api page. But I couldn’t do anything right.

I created IAM User, Role, Policy. I couldn’t pass Step 6 on https://developer-docs.amazon.com/sp-api/docs/creating-and-configuring-iam-policies-and-entities.

I’m developing a desktop application in C#. My goal is for the seller to send and update their products and track their orders. But I couldn’t get anywhere from the documentation. I’ve been trying for a year but I’m tired of it. I need to go through these steps. Please help me.

Good work.