That would suggest you didn’t complete step 3 of oAuth and exchange the code for a token.
3) On your server, get an access token by making this request:
POST https://id.twitch.tv/oauth2/token
?client_id=<your client ID>
&client_secret=<your client secret>
&code=<authorization code received above>
&grant_type=authorization_code
&redirect_uri=<your registered redirect URI>
Here is a sample request:
POST https://id.twitch.tv/oauth2/token
?client_id=uo6dggojyb8d6soh92zknwmi5ej1q2
&client_secret=nyo51xcdrerl8z9m56w9w6wg
&code=394a8bc98028f39660e53025de824134fb46313
&grant_type=authorization_code
&redirect_uri=http://localhost