When I run ionic build android command in root of ionic project, I am getting this error:
FAILURE: Build failed with an exception.
I fix this by downloading sdk package called platform-tools and buid-tools using sdkmanager. You can use sdkmanager.exe or if you are using SDK CLI, go to ~\AppData\Local\Android\sdk\tools\bin and run this command:
sdkmanager "platform-tools" "platforms;android-26"
or
sdkmanager "build-tools;27.0.3"
or both
After that you should be able to run ionic cordova run android or ionic build android.
Note: globalize sdkmanager command by adding ~\AppData\Local\Android\sdk\tools and ~\AppData\Local\Android\sdk\tools\bin to your environment variable.