Well this issue is appearing due to two reasons :
1. permissions to access files from project were not given while running that command
2. or android platform is not installed .
follow these steps :
1. First of all if you are on windows Os run your command prompt as an administrator :
http://www.thewindowsclub.com/how-to-run-command-prompt-as-an-administrator
https://www.howtogeek.com/194041/how-to-open-the-command-prompt-as-administrator-in-windows-8.1/
if you are on mac then try adding ' sudo ' in the beginning of each command .
2. Navigate to your ionic project folder .
Then run following commands :
ionic cordova platform rm android
ionic cordova platform add android
Then generate the resources :
ionic cordova resources
then generate apk .
