The CMake FetchContent mechanism is an anti-pattern, and even if this project supports it - its use should not be recommended. CMake should be used to declare what a project needs, not give instructions on where to take it from. It is the user of the project (e.g. the developer who's building it) that is responsible for making the dependencies available - either via OS distribution packages; manually downloading, configuring, installing and setting a search path; or by using a provisioning manager like conan or vcpkg (which produces a CMake toolchain file).
See also this StackOverflow Q&A.
The CMake FetchContent mechanism is an anti-pattern, and even if this project supports it - its use should not be recommended. CMake should be used to declare what a project needs, not give instructions on where to take it from. It is the user of the project (e.g. the developer who's building it) that is responsible for making the dependencies available - either via OS distribution packages; manually downloading, configuring, installing and setting a search path; or by using a provisioning manager like conan or vcpkg (which produces a CMake toolchain file).
See also this StackOverflow Q&A.