This endpoint is not available in Helix so you will have to use Kraken for now.
It is a v5 API so it requires the numeric channel ID instead of the channel name: https://api.twitch.tv/kraken/chat/125559178/rooms
Also make sure you are sending the correct Accept: application/vnd.twitchtv.v5+json version header along with the request.
{
"_total": 1,
"rooms": [
{
"_id": "2b594b1c-0fa1-4e67-88ea-9518c0d43103",
"owner_id": "125559178",
"name": "subscriber-room",
"topic": "Create a place for your subscribers to hang out",
"is_previewable": false,
"minimum_allowed_role": "SUBSCRIBER"
}
]
}