I found the answer.  I did not know about etags.  Once I included the etags in the complete request, it worked.
I am using the awssdk.s3 library for .Net Core to connect to ECS.  This works in every scenario I have tried except for one.  When I call the CompleteMultipartUploadAsync method, if fails every time.  I know I am using it right, first initiating the multipart upload and then uploading a couple of parts before finally trying to complete it.  Everything leading up to CompleteMultipartUploadAsync works, even the call I make to AbortMultipartUploadAsync after catching the exception.  The ErrorCode of the exception is "InvalidRequest".  Is CompleteMultipartUploadAsync  supported in ECS?  Thank you.
CompleteMultipartUploadAsync Fails