If you are installing appium using node, you may find the following optional dependencies missing while running the command appium-doctor --android:
### Diagnostic for optional dependencies starting ###
WARN AppiumDoctor ✖ opencv4nodejs cannot be found.
WARN AppiumDoctor ✖ ffmpeg cannot be found
WARN AppiumDoctor ✖ mjpeg-consumer cannot be found.
WARN AppiumDoctor ✖ bundletool.jar cannot be found

How to install those manually?
1) opencv4nodejs
npm install -g windows-build-tools --vs2015
npm install -g opencv4nodejs

2) ffmpeg
Go to link https://www.ffmpeg.org/ and download the software
Update the PATH variable with the path to bin folder for example pathto/ffmpeg-20200110-3d894db-win64- static/bin

3) mjpeg-consumer
npm install -g mjpeg-consumer

4) bundletool.jar
Download the jar file from https://github.com/google/bundletool/releases
Rename the jar file as bundletool.jar
Update the PATH environment variable with the path to the file bundletool.jar
Add the ".jar" extension into your PATHEXT environment variable.
That's all, now run the appium-doctor --android command again. This time you the above warnings should go.
info AppiumDoctor Appium Doctor v.1.13.0
info AppiumDoctor ### Diagnostic for necessary dependencies starting ###
info AppiumDoctor ✔ The Node.js binary was found at: <local path>
info AppiumDoctor ✔ Node version is 10.16.3
info AppiumDoctor ✔ ANDROID_HOME is set to: <local path>
info AppiumDoctor ✔ JAVA_HOME is set to: <local path>
info AppiumDoctor ✔ adb exists at: <local path>
info AppiumDoctor ✔ android exists at: <local path>
info AppiumDoctor ✔ emulator exists at: <local path>
info AppiumDoctor ✔ Bin directory of %JAVA_HOME% is set
info AppiumDoctor ### Diagnostic for necessary dependencies completed, no fix needed. ###
info AppiumDoctor
info AppiumDoctor ### Diagnostic for optional dependencies starting ###
info AppiumDoctor ✔ opencv4nodejs is installed at: <local path> Installed version is: 5.5.0
info AppiumDoctor ✔ ffmpeg is installed at: <local path>
info AppiumDoctor ✔ mjpeg-consumer is installed at: <local path>
info AppiumDoctor ✔ bundletool.jar is installed at: <local path>
info AppiumDoctor ### Diagnostic for optional dependencies completed, no fix possible. ###
info AppiumDoctor
info AppiumDoctor Everything looks good, bye!