I am unable to create new asset, getting error 403 - youtube-api

Question

I am trying to enable monetization of a video but I'm getting error 403 and no more additional info
$asset = new \Google_Service_YouTubePartner_Asset();
$metadata = new \Google_Service_YouTubePartner_Metadata();
//if (array_key_exists('title', $data)) $metadata->setTitle($data['title']);
//if (array_key_exists('description', $data)) $metadata->setDescription($data['description']);
$metadata->setTitle('test title');
$metadata->setDescription('test desc');
$asset->setMetadata($metadata);
$asset->setType("web");
// Insert the asset resource. Extract its unique asset ID from the API
// response.
$assetInsertResponse = $this->youtube_partner->assets->insert($asset, ['onBehalfOfContentOwner' => $contentOwnerId]);
and the response is always
{
"error": {
"code": 403,
"message": "Forbidden",
"errors": [
{
"message": "Forbidden",
"domain": "global",
"reason": "forbidden"
}
]
}
}
Everything else seems to be working, i.e I can create policies with
$policy = new \Google_Service_YouTubePartner_Policy();
$policyRule = new \Google_Service_YouTubePartner_PolicyRule();
$policyRule->setAction("monetize");
$policy->setRules(array($policyRule));
$policy->setName('test');
$response = $this->youtube_partner->policies->insert($policy);