An error code 400 is a Bad Request - Somehow your client request is malformed/invalid. Also possible, but less likely, the requested part has not been found.
An error code 401 is Unauthorized - Your access token may not be valid and may need to be refreshed. You have not subscribed your client application to a Digi-Key API plan.
An error code 405 is Method Not Allowed - You are making a request in a format that is not expected. Example is making a GET request when the data needs to be presented as POST.
An error code 429 is Too Many Requests - You have sent too many requests in a given period of time.