Hey Mike - no worries. I was able to do a little bit of digging on my end and based on the logs associated with that X-Request ID you shared the endpoint that was called was :“/admin/api/2022-07/orders/[ordernumber]/fulfillment.json”
Could you try using fulfillments.json instead of fulfillment.json as the endpoint and let us know if that works? This is the only reason why I could think you’d receive a 400 error on this as the rest of your info appears to be correct.
Hope this helps - as always let us know if we can clarify anything further on our end. Cheers!