该问题可能是由于 MediaConvert 对输入文件中的媒体和容器类型不兼容而导致的。为了避免出现此错误，应在作业设置中指定合适的容器和媒体类型。

可以通过以下代码示例，指定输入文件的媒体类型为“video/mp4”和容器类型为“mp4”：

var importSettings = new ImportSettings
{
    Format = "mp4",
    FileInput = "s3://input-bucket/input-file.mp4",
    VideoSelector = new VideoSelector
    {
        ColorSpace = "FOLLOW",
        ColorSpaceUsage = "FORCE",
        SelectorSettings = new VideoSelectorSettings
        {
            FileSourceConvert608To708 = "DISABLED",
            CaptionSelectorSettings = new CaptionSelectorSettings
            {
                LanguageCode = "en",
                SourceSettings = new CaptionSourceSettings
                {
                    SourceType = "ANCILLARY"
                }
            }
        }
    },
    AudioSelectors = new Dictionary
    {
        ["Audio Selector 1"] = new AudioSelector
        {
            DefaultSelection = "DEFAULT",
            AudioSelectorSettings = new AudioSelectorSettings
            {
                AudioTrackSelection = new AudioTrackSelection
                {
                    Tracks = [0],
                    TrackType = "AUDIO"
                }
            }
        }
    }
};

设置好媒体和容器类型后，再执行转换操作，应该能够成功转换 MP4 文件了。