Read the body of the response not just the HTTP code.
That’ll tell you the issue.
You will find the response is
{
error: 'Bad Request',
status: 400,
message: 'Missing required parameter "reward_id"'
}
A reward_id is required.
When it says When ID is not provided it means the optional query string parameter of id
IE you either look up redeems by up to 50 single redeem ID’s or by one rewardID
Consider using EventSub to be notified of a redemption instead of long polling