A Question Regarding TikZ ellipse
When using ellipse as a node shape, you need to load the shapes.geometric tikz library.
You can do that by adding \usetikzlibrary{shapes.geometric} in your preamble (usually immediately) after \usepackage{tikz}.
Note that you don't need shapes.geometric to draw an ellipse, only to use it as a node shape like in your example.
If you use several shapes.xxx libraries, you can simply write \usetikzlibrary{shapes} instead of listing them one by one.
As supplement to the Étienne answer:
- Tik Z & PGF Manual for Version 2.10-CVS is obsolete for years, I strongly recommend to use the recent version of the package as well as its manual.
- Actual version of TikZ manual is 3.1.5b (January 8, 2020), where your example is on page 141.
- Shapes Library is described in chapter 72 Shape Library starting at page 785, where in section 72 Shape Library is stated:
In addition to the standard shapes rectangle, circle and coordinate, there exist a number of additional shapes defined in different shape libraries. Most of these shapes have been contributed by Mark Wibrow. In the present section, these shapes are described. Note that the library shapes is provided for compatibility only. Please include sublibraries like shapes.geometric or shapes.misc directly.
- Geometric shapes are described in section 72.3 Geometric Shapes on page 786.
- For ellipse shape see pages 787 -- 788.