I get this error too and fretted for a long time : (

After a bunch of google searches , I found some solutions that may help.

At least it works on my Ubuntu 20.04 VM.

( The instruction to download Android Studio and copy-paste the dir part ( last 3 steps ) may solve your problem , as I also got the same error as you )

• first update apt

  sudo apt-get update
  

• upgrade pkgs

  sudo apt-get upgrade
  

• install python3 + pip

  sudo apt-get install python3 python3-pip
  

• config default Python version ( ref_1 ) ( ref_2 )

  sudo update-alternatives --install /usr/bin/python python /usr/bin/python<X.X.X> 1
  sudo update-alternatives --config python
  

  where <X.X.X> is the desired Python ver.

• install dev tools + dependencies

  sudo apt-get install build-essential \
    libssl-dev \
    libffi-dev \
    python3-dev \
    dh-autoreconf \
    autoconf \
    libtool \
    pkg-config \
    zlib1g-dev \
    libncurses5-dev \
    libncursesw5-dev \
    libtinfo5 \
    cmake \
    ccache
  

• install kivy

  • through apt

    sudo apt-get install python3-kivy
    

  • through Python pip ( my preferred way )

    python3 -m pip install kivy==2.0.0
    

• install cython

  pip3 install Cython
  sudo apt-get install cython
  

  // I do it in this way

• install javac

  sudo apt-get install openjdk-11-jdk openjdk-8-jdk
  

• check if java and javac are installed

  java -version
  javac -version
  

• config. java + javac ( I config. it to openjdk8-jdk )

  sudo update-alternatives --config java
  

  sudo update-alternatives --config javac
  

• install cmake ( I did both of it ) ( ref )

  • python way

    pip3 install cmake
    

  • manual way

    cd /tmp
    wget https://github.com/Kitware/CMake/releases/download/v3.20.0/cmake-3.20.0.tar.gz
    tar -zxvf cmake-3.20.0.tar.gz
    cd cmake-3.20.0
    ./bootstrap
    make
    sudo make install
    

• install buildozer ( I reinstalled it using the official way )

  • official way

    pip3 install --user --upgrade buildozer
    

  • through git clone

    sudo apt-get install git
    git clone https://github.com/kivy/buildozer.git
    cd buildozer
    sudo python3 setup.py install
    

• install Python IDLE ( optional ) ( either one of the following )

  sudo apt-get install idle3
  

• install Android Studio

  I tried the way not to install Android Studio ( using cmdline-tools only ), but just can't figure it out : (

  sudo snap install android-studio --classic
  

  and run it and let it set up everything by itself.

  After that, just close it

• after everything is set up : ( ref )

  open file

  copy the tools dir located in ~/Home/Android/Sdk/

  paste it at ~/Home/.buildozer/android/platform/android-sdk after compressing the original tools dir located there ( by right click --> compress ) ( if you can't see the dir, make sure to check the show hidden file checkbox in the file's option )

  then go to ~/Home/.buildozer/android/platform/android-sdk/tools/bin and open in terminal

  ./sdkmanager --install "tools"
  exit
  

• rebuild the project again go to your project's dir

  buildozer init  # if you haven't got the buildozer.spec in your project's dir
  buildozer android debug
  

  *.apk will be available in bin

my buildozer.spec:

title = Screen_Recorder
package.name = screen_recorder
package.domain = org.test
souce.dir = .
source.include_exts = py,png,jpg,kv,atlas
requirements = python3,kivy==2.0.0,android,opencv==4.5.3,numpy,pillow,EasyProcess,entrypoint2,mss,jeepney,plyer,pyscreenshot
osx.python_version = 3
osx.kivy_version = 2.0.0
android.permission = CAMERA,RECORD_AUDIO,WRITE_EXTERNAL_STORAGE,READ_EXTERNAL_STORAGE

Reference

• Stackoverflow Question: kivy camera application with opencv in android shows black screen
• Stackoverflow Question: Logs Generated when launching an App in Kivy Launcher on Android
• Stackoverflow Question: I Can't build buildozer for android
• Youtube Video (I follow this video (the copy-paste dir part) to make it work): to get opencv4 work with kivy / buildozer on ubuntu
• git discussion: Android compilation fails with opencv
• Stackoverflow Question: android permissions in python using request_permissions
• python-for-android docs - Working on Android

Note

• close the terminal and reopen it if you make changes to the root dir
• don't edit the .bashrc file ( in my case , I didn't )
• delete the bin and .buildozer dir in your project's dir every time you rebuild your project OR run buildozer <platform> clean
• use ls to list all files + folders in the current dir
• use cd <dir> to navigate to directory
• use cd .. to navigate to the previous dir
• if you need permission like CAMERA , you need to add it to the buildozer.spec's permission and add the following in your *.py to gain android permission :

  from kivy.utils import platform
  
      if platform == "android":
  
          from android.permissions import request_permissions, Permission
  
          request_permissions([ <permissions> ])  # e.g. Permission.WRITE_EXTERNAL_STORAGE , Permission.READ_EXTERNAL_STORAGE , Permission.CAMERA , Permission.RECORD_AUDIO , etc.
  

  ( You don't need to install or import android , just include android in the buildozer.spec's requirement )

Hope This Helps ( please correct if you find anything is incorrect )