I was having this issue and the way I worked out to do this:
You need to add android first:
ionic capacitor add android
ionic capacitor copy android && cd android && ./gradlew assembleDebug && cd ..
Then your apk will be at:
android/app/build/outputs/apk/debug/app-debug.apk
If you want to run on device directly from command line:
ionic capacitor copy android && cd android && ./gradlew assembleDebug && ./gradlew installDebug && cd ..
Note: It doesn’t work without entering the android directory
I hope that helps!