Code 21002 means that the JSON you are sending to apple which has your shared secret and your receipt data is "misformed" or not in the format apple wants it.

Also Try removing from receipt the characters '\n' and '\r' and replacing '+' with'%2B' before sending it to the server. Something like this:
NSURL *receiptURL = [[NSBundle mainBundle] appStoreReceiptURL];
NSData *receipt = [NSData dataWithContentsOfURL:receiptURL];
NSString *receiptDataString = [receipt base64EncodedStringWithOptions:0];
receiptDataString=[receiptDataString stringByReplacingOccurrencesOfString:@"+" withString:@"%2B"];
receiptDataString=[receiptDataString stringByReplacingOccurrencesOfString:@"\n" withString:@""];
receiptDataString=[receiptDataString stringByReplacingOccurrencesOfString:@"\r" withString:@""];
NSString *postDataString = [NSString stringWithFormat:@"receipt-data=%@", receiptDataString];
NSString *length = [NSString stringWithFormat:@"%lu", (unsigned long)[postDataString length]];
[request setValue:length forHTTPHeaderField:@"Content-Length"];
[request setHTTPBody:[postDataString dataUsingEncoding:NSASCIIStringEncoding]];
Reference
Code 21004 this means you have a logical error in the code, somehow you are not passing the key in that case