Getting a CMake Error: Cannot specify link libraries for target which is not built by the project

In addition to the accepted answer: An important detail is to place target_link_libraries after the add_executable and find_package lines, so all linked components are known.

The first argument of target_link_libraries is the target name:

target_link_libraries(eCAD Qt5::Widgets Qt5::Core) 

Also, do not confuse target name with the project name:

• a command project specifies a project name, but
• a target is the one created with add_executable, add_library or add_custom_target.

The error message is about the target.