Not all versions of Thonny include the GuiZero Library. You can install it (or other libraries) as follows in Thonny:

  1. Choose Tools → Manage packages…
  2. Enter “guizero” (in the search bar at the top — To install other libraries, replace guizero here with other library names).
  3. Press the “Find package from PyPI” button (on the top right).
  4. Press the “Install” button (at the bottom).
  5. Press “Close”.

You can check your installation by running this sample GuiZero program:

template-drawing.py