During the developer preview of AWS Elemental MediaConvert you have to specify which MediaConvert service you want to use when you use a MediaConvert specific shape-tag. Specify the resource using the resourceId parameter when executing the transcode job. 

POST /API/item/VX-61/transcode?tag=__mediaconvert_Generic_Hd_Mp4_Avc_Aac_16x9_1280x720p_24Hz_4.5Mbps&resourceId=VX-11

In the above case VX-11 is the MediaConvert service in my Vidispine API instance. To find out which service it is in your system, use GET /API/resource/vidinet/, or use the Vidinet Dashboard (see image below).