UWP中的每个UI元素一次只能在UI中的一个地方使用。您需要在每个循环中定义UI元素。

foreach(var btm in bitmapImages)
{
    Canvas page = new Canvas
    {
        Width = pageDescription.PageSize.Width,
        Height = pageDescription.PageSize.Height
    };

    Canvas viewablePage = new Canvas()
    {
        Width = pageDescription.ViewablePageSize.Width,
        Height = pageDescription.ViewablePageSize.Height
    };

    viewablePage.SetValue(Canvas.LeftProperty, pageDescription.Margin.Width);
    viewablePage.SetValue(Canvas.TopProperty, pageDescription.Margin.Height);

    // The image "frame" which also acts as a viewport
    Grid photoView = new Grid
    {
        Width = pageDescription.PictureViewSize.Width,
        Height = pageDescription.PictureViewSize.Height
    };
    Image image = new Image
    {
        Source = btm,
        HorizontalAlignment = Windows.UI.Xaml.HorizontalAlignment.Center,
        VerticalAlignment = Windows.UI.Xaml.VerticalAlignment.Center
    };

    // Use the real image size when croping or if the image is smaller then the target area (prevent a scale-up).
    if ((btm.PixelWidth <= pageDescription.PictureViewSize.Width &&
        btm.PixelHeight <= pageDescription.PictureViewSize.Height))
    {
        image.Stretch = Stretch.None;
        image.Width = bitmap.PixelWidth;
        image.Height = bitmap.PixelHeight;
    }
    photoView.Children.Add(image);
    viewablePage.Children.Add(photoView);
    page.Children.Add(viewablePage);
    pages.Add(page);
}

尝试将Image添加到循环内部的photoView，并在循环外部仅添加一次其他子元素：

foreach (var btm in bitmapImages)
{
    Image image = new Image
    {
        Source = btm,
        HorizontalAlignment = Windows.UI.Xaml.HorizontalAlignment.Center,
        VerticalAlignment = Windows.UI.Xaml.VerticalAlignment.Center
    };

    // Use the real image size when croping or if the image is smaller then the target area (prevent a scale-up).
    if ((btm.PixelWidth <= pageDescription.PictureViewSize.Width &&
        btm.PixelHeight <= pageDescription.PictureViewSize.Height))
    {
        image.Stretch = Stretch.None;
        image.Width = bitmap.PixelWidth;
        image.Height = bitmap.PixelHeight;
    }
    photoView.Children.Add(image);
}

viewablePage.Children.Add(photoView);
page.Children.Add(viewablePage);
pages.Add(page);

如果您仍然遇到相同的错误，请修改您的问题，解释viewablePage、page和page是什么以及如何使用它们。