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The path operations (Union, Difference, etc.) work on shapes or areas, rather than on nodes or paths. So if there's something you want to do to the nodes, it will be some other command. I'm not quite sure what you mean by "union nodes". Do you mean that you want to join them together?
OH! Maybe you want to make them all subpaths of one compound path? If that's it, then select them all, like you've already done, then use Path menu > Combine. This combines all the paths into one. The only problem with that, relative to your screenshot, is that the 2 circles' paths have different widths. When you combine them, they will all take the same style, which includes width. If you need to keep them with different widths, then before you combine them, you'll need to select one of the circles, and do Path menu > Stroke to Path.
Or maybe I have not guessed what you mean by "union nodes" yet?