Я получаю эту ошибку, потому что я не установил параметр в правильном формате. Теперь я использую этот код и работает нормально.
NSString * trimmedText = [NSString stringWithFormat: @ "% @", txt_comment.text];
NSURL *url=[NSURL URLWithString: @"https://api.twitter.com/1/statuses/update.xml"];
NSMutableDictionary *params = [NSMutableDictionary dictionaryWithCapacity:0];
[params setObject:trimmedText forKey:@"status"];
NSString *body = [self _queryStringWithBase:nil parameters:params prefixed:NO];
NSData* requestData = [body dataUsingEncoding: NSUTF8StringEncoding];
OAMutableURLRequest *request = [[[OAMutableURLRequest alloc] initWithURL:url consumer: appDelegate.consumer token: appDelegate.accessToken realm: nil signatureProvider: nil] autorelease];
[request setHTTPMethod:@"POST"];
[request setHTTPBody:requestData];
OADataFetcher *fetcher = [[OADataFetcher alloc] init];
[fetcher fetchDataWithRequest:request
delegate:self
didFinishSelector:@selector(accessTokenTicket_post:didFinishWithData:)
didFailSelector:@selector(accessTokenTicket:didFailWithError:)];