The simple answer is that pico_add_extra_outputs is defined in the pico sdk and your CMakeLists.txt file doesn't include a reference to it.

There's nothing wrong with the CMake file, but you need to understand a little about how CMake works to see why the problem occurred. The directory that you're working from contains the source and a CMakeLists.txt file for this subproject. This file isn't a complete cmake file and is intended to be included by parent directory's CMakeLists.txt file. This parent file will include the pico sdk and initialize it before bringing each of the subprojects through add_subdirectory instructions.

An easy way to try out the individual examples is through the VSCode pico extension. Its new project function enables the creation of a project for the sample that you're interested in or build your own from a range of features. You don't need to configure cmake and the.projects are ready to build, debug and deploy.