Troubleshooting Purchase Order Creation via Postman API Integration

I'm encountering an issue while attempting to create a purchase order through the API using Postman. Here's how I've proceeded: first, I generated an access token using Resource Owner Password Credentials. Then, I exchanged this token for a code to proceed with the purchase order creation. In the request body, I included a default raw dict that I found in the Acumatica discussion forum.

However, despite following these steps, I'm encountering an error. Could someone please offer guidance on how to resolve this issue? Any help or suggestions would be greatly appreciated.

This is how i have successfully generated the Access Token 
 

Now when i use this exchange token to create an PO in the acumatica using an API 
 

This is here i have put the access token and this is the API that i an using to create an PO in the acumatica. 
In the body i have used this raw dictionary that i found in the acumatica discussions form 
 

And in the headers i am using these 
 

After doing all this i am getting this error 
 

However, despite following these steps, I'm encountering an error. Could someone please offer guidance on how to resolve this issue? Any help or suggestions would be greatly appreciated.