undefined reference to `cv::face::FisherFaceRecognizer::create'

This error message typically occurs when the linker is unable to find the implementation of the `cv::face::FisherFaceRecognizer::create` function.

To resolve this issue, you can try the following steps:

1. Make sure that you have included the necessary headers in your code. You should include the following header files:

#include <opencv2/face.hpp>
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>

2. Check that you have linked the necessary libraries. You should link against the following libraries:

-lopencv_core
-lopencv_face
-lopencv_highgui

3. Confirm that you have the correct version of OpenCV installed. The `FisherFaceRecognizer` class is only available in OpenCV version 3.0 and above.

4. If you are using CMake, make sure that you have added the necessary libraries to your `CMakeLists.txt` file. You can add the following lines to your `CMakeLists.txt` file:

find_package(OpenCV REQUIRED)
target_link_libraries(your_target_name ${OpenCV_LIBS})

By following these steps, you should be able to resolve the `undefined reference to cv::face::FisherFaceRecognizer::create` error.