When adding animation cels, you must add a cel layer inside an animation folder, and then [Specify cel] on the timeline palette.

You cannot draw on a layer without [Specify cel], even if it is placed inside an animation folder.
This applies to images that are imported by scanning, etc. The image will not be displayed on the canvas until the layer is specified with [Specify cel].

Solution

1. Confirm that the layer is in the [Animation folder].

2. In the [Animation folder] row of the timeline palette, right-click the position where you want to display the cel. Then, select the name of the layer you added, and set [Specify cel].

*For iPad users, long press the icon.

3. You can now draw on the layer you added.
If the image was scanned and imported from outside, the image is displayed on the canvas.

The [Specify cel] feature of the timeline can specify a layer folder as one cel.
This is useful when you want to separate the line art and painting layer.