I can't get spc when use Fairplay in IOS

streamingContentKeyRequestDataForApp:contentIdentifier function always return error 268451843

[1]@"NSLocalizedFailureReason" : @"An unknown error occurred (268451843)"

[2]@"NSLocalizedDescription" : @"The operation could not be completed"

- (NSData *)myGetAppCertificateData
{
     NSString *resourcePath = [[NSBundle mainBundle] pathForResource:@"fairplay" ofType:@"der"];
    NSData*certificate=[NSData dataWithContentsOfFile:resourcePath];
    return certificate;
}

in resourceLoader:(AVAssetResourceLoader *)resourceLoader shouldWaitForLoadingOfRequestedResource:(AVAssetResourceLoadingRequest *)loadingRequest

  NSString *assetStr;
  NSData *assetId;
  NSData *requestBytes;


    assetStr = [url host];
    assetId = [NSData dataWithBytes: [assetStr cStringUsingEncoding:NSUTF8StringEncoding] length:[assetStr lengthOfBytesUsingEncoding:NSUTF8StringEncoding]];
    NSData *certificate = [self myGetAppCertificateData];
    #pragma mark ADAPT: YOU MUST CALL: streamingContentKeyRequestDataForApp::options:


    requestBytes = [loadingRequest streamingContentKeyRequestDataForApp:certificate
                                                      contentIdentifier:assetId
                                                                options:nil
                                                                  error:&error];

please help us to debug