Hi, you need to "link" SMFL to your project. There are three things to do
1- In CodeBlocks go to Project/Build options/Search directories/Compiler and add the path to sfml include
folder
2- Go to Project/Build options/Search directories/Linker and add the path to sfml lib
folder
3- Go to Project/Build options/Linker settings, on the left "Link libraries" add the list of sfml libs.
The order is important, look at the picture attached