I was unable to send a request to Airtable using ReqBin, but I was able to successfully send the exact same request to Airtable using both Postman and Paw.
So perhaps ReqBin is sending the request in a non-standard way that Airtable can’t recognize. I would try using Postman or Paw or some other tool instead.
Also, you may want to report the ReqBin problem to support@airtable.com, in case it’s a problem on Airtable’s end.