To convert audio file from .wav to .mp3 using ffmpeg flutter

I needed to convert an audio file from .wav to .mp3, i have used this package for converting.But its returning with code 1.

 FFmpegKit.execute('-i $url output.mp3').then((session) async {
                                                        final returnCode = await session.getReturnCode();

                                                        if (ReturnCode.isSuccess(returnCode)) {


                                                          // SUCCESS

                                                        } else if (ReturnCode.isCancel(returnCode)) {

                                                          // CANCEL

                                                        } else {


                                                          // ERROR

                                                        }
                                                      })

conversion no