Build fails. [Error] : ‘xfeatures2d’ has not been declared`

[adbidwai]

System configurations
OS: Ubuntu 18.04
ROS Distro: ROS Melodic
When I try to build the packages explore and map_merge I get the following error.

Errors     << multirobot_map_merge:make /home/adbidwai/nus_ws/logs/multirobot_map_merge/build.make.003.log      
In file included from /home/adbidwai/nus_ws/src/m-explore/map_merge/src/combine_grids/merging_pipeline.cpp:46:0:
/home/adbidwai/nus_ws/src/m-explore/map_merge/src/combine_grids/estimation_internal.h: In function ‘cv::Ptr<cv::Feature2D> combine_grids::internal::chooseFeatureFinder(combine_grids::FeatureType)’:
/home/adbidwai/nus_ws/src/m-explore/map_merge/src/combine_grids/estimation_internal.h:68:14: error: ‘xfeatures2d’ has not been declared
       return xfeatures2d::SURF::create();
              ^~~~~~~~~~~
make[2]: *** [CMakeFiles/combine_grids.dir/src/combine_grids/merging_pipeline.cpp.o] Error 1
make[1]: *** [CMakeFiles/combine_grids.dir/all] Error 2
make: *** [all] Error 2
cd /home/adbidwai/nus_ws/build/multirobot_map_merge; catkin build --get-env multirobot_map_merge | catkin env -si  /usr/bin/make --jobserver-fds=6,7 -j; cd -
................................................................................................................
Failed     << multirobot_map_merge:make              [ Exited with code 2 ]                                     
Failed    <<< multirobot_map_merge                   [ 17.4 seconds ]

My system has an OpenCV version as '4.2.0'.

 adbidwai@adbidwai-g3-3579  ~  pkg-config --modversion opencv4
4.2.0

What dependencies is my system missing?

Any help is appreciated,
TIA

[adbidwai]

For reference :

 adbidwai@adbidwai-g3-3579  ~  python                         
Python 2.7.17 (default, Feb 27 2021, 15:10:58) 
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
>>> cv2.__version__
'4.2.0'

 adbidwai@adbidwai-g3-3579  ~  python3
Python 3.6.9 (default, Jan 26 2021, 15:33:00) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
>>> cv2.__version__
'4.2.0'