Hey there, this is because we don’t yet allow all API key routes to be called via HttpService.
Refer to: [Beta] Use Open Cloud via HttpService Without Proxies
The feature is still in beta so the amount of allowed Roblox endpoints you can hit from Studio / in-experience is limited. We will expand it in the future.
Please also note that you must use Secret Store to store the API key you send along to HttpService, it will not work if you pass it like a string like this.