.Net core and Sales Force Rest api integration

Hello Professionals,
I am a .Net developer and new to the salesforce api’s.
I have developed a console application that will perform CRUD operations on Lead object.
I am able to get the access token, and I could read the lead data. While I try to add or delete the lead record I get an error. I have attached an error and snap of the code.
I spent around 3 hours to resolve the error but not success. I would need your help to proceed ahead with the taskI have followed blog of Prasad Ruwanapattirana.https://www.forcetalks.com/blog/integrate-net-console-application-with-salesforce-rest-api/