[How To] Impossible to fill this kind of level

Merry Christmas to you!
Now, I don't peak Italian, but from the photo, I am assuming that "livello" means layer. So it's a wrong type of layer.

Your highlighted (selected) layer seems to be an image, it cannot be filled...

Try clicking on an empty layer and fill it from there (after all you wouldn't want to fill where your drawing is!)

see my screenshot below

I cannot fill that type of layer as it's considered an image.

See the little (Very small) icon next to the layer, it looks like a mountain (symbol for image)

Now try to draw with a vector brush (which will create a new vector layer) and see how the icon next to it is different, and same with a standard brush, the icon will look different too:

You can fill the last two, but not a background or an image layer.

Hope I explained it well 🙂