Yes, here you have only asked for the scopes
openiduser_read
user_read is a “dead” scope and no longer documented for use
You didn’t include the broadcaster_id in the query string parameters
So you did https://api.twitch.tv/helix/streams/key
Instead of https://api.twitch.tv/helix/streams/key?broadcaster_id=xxxxx
where xxxxx is the ID of the caster you wish to get the stream key of