Were you able to resolve this issue?
Anyhow I present a solution that might help.
The open-cv package requires some additional packages when being installed in raspberry pi(I’m using model 4B).
pip3 install opencv-python 
sudo apt-get install libcblas-dev
sudo apt-get install libhdf5-dev
sudo apt-get install libhdf5-serial-dev
sudo apt-get install libatlas-base-dev
sudo apt-get install libjasper-dev 
sudo apt-get install libqtgui4 
sudo apt-get install libqt4-testv
echo "deb https://packages.cloud.google.com/apt coral-edgetpu-stable main" | sudo tee /etc/apt/sources.list.d/coral-edgetpu.list
curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -

Installing these dependencies would make cv2 work on your RPi.

Chances are you will face the same issue when using opencv for object detection using a Linux(I use Ubuntu 20.04 LTS) system where you will get an error like :-
cv2.error: OpenCV(3.4.2) /tmp/build/80754af9/opencv-suite_1535558553474/work/modules/highgui/src/window.cpp:632: error: (-2:Unspecified error) The function is not implemented. Rebuild the library with Windows, GTK+ 2.x or Carbon support. If you are on Ubuntu or Debian, install libgtk2.0-dev and pkg-config, then re-run cmake or configure script in function 'cvShowImage'

This can be resolved by following the steps:-
 Dont install opencv-python, install from the source
To get over this error uninstall opencv-python and reinstall it using source following the tutorial below:
  
      

      linuxize.com – 5 Jul 20
  

  
    

How to Install OpenCV on Ubuntu 20.04

  This article describes how to install OpenCV on Ubuntu 20.04. OpenCV is an open-source computer vision library with bindings for C++, Python, and Java.


  

  
    
    
  

  

During the setup of opencv in the build in addition to the given configs also add -D WITH_QT=ON -D WITH_GTK=ON 
This basically installs the GUI backend for running Opencv in Linux sytems
Hopefully it will start working fine after that