You need to request access to Advertising API in the products tab:

Once you have access the this API you will see other permissions added to your OAuth 2.0 scopes tab of you app.

After that, you need to generate a new Access Token using OAuth 2.0 tools ( link below ) :

https://www.linkedin.com/developers/tools/oauth?clientId={YOUR_CLIENT_ID}

Finally you can send the request to post using Bearer Token Auth Type.

I hope that helps.