URLからスペースを置き換えるための正しい形式は次のとおりです。
Swift 4.2、Swift 5
var urlString = originalString.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)
スウィフト4
var urlString = originalString.addingPercentEncoding(withAllowedCharacters: CharacterSet.urlQueryAllowed)
Objective C
NSString *urlString;//your url string.
urlString = [originalUrl stringByReplacingOccurrencesOfString:@" " withString:@"%20"];
また
urlString = [originalUrl stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
iOS9以降
urlString = [originalUrl stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet URLQueryAllowedCharacterSet]];