Stupid question, but does your service account automatically have access to your youtube account? It is not obvious to me that it would.
I think you may be better off getting credentials via a regular oauth flow, which is what the api explorer does.
Here i have implementd a modified version of the web quickstart from the youtube python api examples, which presents a webview that authorizes to a flask webserver. Then saves the access token to creds.json.
This requires you to add http://localhost:8090/oauth2callback to the allowed callback uris in the developer console.
Also, a bunch of packages need to get installed via stash pip --
Once you have the token for your user, you can use the google youtube python api. I started with the youtube/api-samples repo on github, replaced their get_authenticated_service with one that uses the saved creds.json.
# Authorize the request and store authorization credentials.
from oauth2client.client import AccessTokenCredentials
return build(API_SERVICE_NAME, API_VERSION, credentials = credentials)
After that, I was able to create a playlist.