Set up your CMake project before selecting a target.

[PTSDenjoyer8]

I am new to CMake and when I want to build the CMakeLists.txt it keeps saying: "Set up your CMake project before selecting a target".
The code I have in CMakeLists.txt is:
cmake_minimum_required(VERSION 3.0)

project(Detection)

set(OpenCV_DIR D:/OpenCV/opencv/build)
find_package(OpenCV REQUIRED)

add_executable(${PROJECT_NAME} firstDetection.cpp)

target_link_libraries(${PROJECT_NAME} ${OpenCV_LIBS})

I saw these on YouTube and the guy built it without any errors