I would like to organize my Google Keep notes, using the labels feature. Is there any way to show only notes without a label?
Nope, doesn't appear to be so. You should use the "feedback" feature to let them know that that's something you want.
The closest thing available is that you can filter by color. (Color used to be the only way to organize your notes.) If you're consistent about applying a color to items that you've also labeled, you can search for color of "None".
(For what it's worth, the Android app is the same.)
There is no direct way, but there is sideway: use colors. Select color which you will left for not labeled notes. Check that for that colors you don't have notes with labels. If you have such: re-color them. For every label you have, select all notes of that label and assign some color. Finally, you will have all unlabeled notes grouped by the selected color.