the error you get is telling you that it cannot find opencv include file:

#include <opencv2/core.hpp>

you need to locate the opencv2 folder, and pass it to cmake as an additional include directory:

https://cmake.org/cmake/help/v3.0/command/include_directories.html

example: Adding OpenCV to Native C code through CMake on Android Studio

cmake_minimum_required(VERSION 3.4.1)
set(OpenCV_DIR "src/sdk/native/jni")
find_package(OpenCV REQUIRED)
message(STATUS "OpenCV libraries: ${OpenCV_LIBS}")
include_directories(${OpenCV_INCLUDE_DIRS})

or you can use this approach:
https://github.com/ahasbini/Android-OpenCV/blob/master/app/CMakeLists.txt

It looks like your cmake project couldn’t find the installed opencv, so either you set your path to ${complete_path_to_opencv_installation}/apt or you install opencv in common installation folder

Either way, you’ll need to propagate this to your project:
In case you install this in typical place (e.g. /usr/local/ in linux you’ll need to set export PATH=$PATH:${opencv_installation_path} or c:Path in Windows, then set Path in Environment Variable.

By doing so, the find_package(OpenCV) should be able to find the opencv library and OpenCV_INCLUDE_DIRS is set.
This CMakeLists.txt should work:

cmake_minimum_required(VERSION 3)
project( DisplayImage )
find_package( OpenCV REQUIRED )
include_directories( ${OpenCV_INCLUDE_DIRS} )
add_executable( DisplayImage DisplayImage.cpp )
target_link_libraries( DisplayImage ${OpenCV_LIBS} )

or you could also set OpenCV_INCLUDE_DIRS "./app/opencv2" before include_directories
to propagate opencv2/core.hpp.

You could also interested to this older post about using opencv with android.

I have custom C++ classes which I want to run on my android app. I am successfully binded my C++ files using CMakeLists. But as my classes uses opencv, I am getting an issue which is obvious

fatal error: 'opencv2/core.hpp' file not found

Then I tried to add opencv library on my android app, downloaded the open cv android sdk and tired to add it on my project. Below is the folder structure

android 
   -> app
       -> opencv2
          All the opencv2 c++ files and folders
       ->folder1
          My custom c++ classes which will import opencv2.

The following is my CMakeLists.txt file

cmake_minimum_required(VERSION 3.4.1)  # for example

set(CMAKE_TRY_COMPILE_TARGET_TYPE "STATIC_LIBRARY")

PROJECT(tag)


set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")

 add_library( tag_native
              # Sets the library as a shared library.
              SHARED
              # Provides a relative path to your source file(s).
              "./folder1/tag_native.cpp" )


add_library( opencv2
             # Sets the library as a shared library.
             SHARED
             # Provides a relative path to your source file(s).
             "./opencv2/" )

set_target_properties(opencv2 PROPERTIES LINKER_LANGUAGE CXX)

This is the sample of my cpp file

#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <vector>
#include <iostream>
#include <opencv2/core.hpp>

Everything from c++ compiles well but I am still getting the same issue

* What went wrong:
Execution failed for task ':app:externalNativeBuildDebug'.
> Build command failed.
  Error while executing process C:UsersBrainants with arguments {TechnologyAppDataLocalAndroidSdkcmake3.10.2.4988404binninja.exe -C E:SqutagSqutag Mobileandroidapp.cxxcmakedebugarmeabi-v7a opencv2 squtag_native}
  ninja: Entering directory `E:SqutagSqutag Mobileandroidapp.cxxcmakedebugarmeabi-v7a'
  [1/2] Building CXX object CMakeFiles/squtag_native.dir/E_/Squtag/Squtag_Mobile/ios/Runner/Squtag_Native/squtag_native.cpp.o
  FAILED: CMakeFiles/squtag_native.dir/E_/Squtag/Squtag_Mobile/ios/Runner/Squtag_Native/squtag_native.cpp.o 
  C:UsersBRAINA~1AppDataLocalAndroidSdkndk210~1.611TOOLCH~1llvmprebuiltWINDOW~1binCLANG_~1.EXE --target=armv7-none-linux-androideabi24 --gcc-toolchain="C:/Users/Brainants Technology/AppData/Local/Android/Sdk/ndk/21.0.6113669/toolchains/llvm/prebuilt/windows-x86_64" --sysroot="C:/Users/Brainants Technology/AppData/Local/Android/Sdk/ndk/21.0.6113669/toolchains/llvm/prebuilt/windows-x86_64/sysroot"  -Dsqutag_native_EXPORTS  -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 -march=armv7-a -Wformat -Werror=format-security  -fsanitize=address -fno-omit-frame-pointer -std=c++11 -O0 -fno-limit-debug-info  -fPIC -MD -MT CMakeFiles/squtag_native.dir/E_/Squtag/Squtag_Mobile/ios/Runner/Squtag_Native/squtag_native.cpp.o -MF CMakeFilessqutag_native.dirE_SqutagSqutag_MobileiosRunnerSqutag_Nativesqutag_native.cpp.o.d -o CMakeFiles/squtag_native.dir/E_/Squtag/Squtag_Mobile/ios/Runner/Squtag_Native/squtag_native.cpp.o -c "E:/Squtag/Squtag Mobile/ios/Runner/Squtag Native/squtag_native.cpp"
  E:/Squtag/Squtag Mobile/ios/Runner/Squtag Native/squtag_native.cpp:6:10: fatal error: 'opencv2/core.hpp' file not found
  #include <opencv2/core.hpp>
           ^~~~~~~~~~~~~~~~~~
  1 error generated.
  ninja: build stopped: subcommand failed.