OAuth Authentication Fix for Amazon SP API

I have an issue with the OAuth integration in my Amazon SP API application. The specific problem is an authentication failure that typically occurs during the initial login. This issue arose without any recent changes to the application or its settings.
I recieved 2 params seller id and state but unable to find SPAPI oauth code.

Ideal skills and experience for the job:
- In-depth knowledge of Amazon SP API
- Proven experience in resolving OAuth issues
- Strong background in application development and troubleshooting