Android SDK error even after installing it Ionic framework

I have have been trying to setup the environment to work with ionic and have follwed all the steps here

https://blog.nraboy.com/2014/09/install-android-cordova-ionic-framework-ubuntu/

after that i was getting the error for the android SDK so i installed it but still having the same error

[Error: Please install Android target: "android-21".

Hint: Open the SDK manager by running: /opt/android-sdk/tools/android
You will require:
1. "SDK Platform" for android-21
2. "Android SDK Platform-tools (latest)
3. "Android SDK Build-tools" (latest)]
ERROR building one of the platforms: Error: /home/tasol/todo/platforms/android/cordova/build: Command failed with exit code 2
You may not have the required environment or OS to build this project
Error: /home/tasol/todo/platforms/android/cordova/build: Command failed with exit code 2
    at ChildProcess.whenDone (/usr/lib/node_modules/cordova/node_modules/cordova-lib/src/cordova/superspawn.js:131:23)
    at ChildProcess.emit (events.js:98:17)
    at maybeClose (child_process.js:766:16)
    at Process.ChildProcess._handle.onexit (child_process.js:833:5)

You can see in the attached screenshot that all the packages are installed then also i am facing the same issue.

Can anyone please help me. Thanks in advance.

Using: Ubuntu 14.04

I have got rid of that error by following Tim's answer as i am a php developer and this is my first time with android sdk sorry if i sounds nube

The above error solved with the answer provided by Tim but now i am facing some new errors so updating the answer with them

ERROR : No emulator images (avds) found.
1. Download desired System Image by running: /opt/android-sdk/tools/android sdk
2. Create an AVD by running: /opt/android-sdk/tools/android avd
HINT: For a faster emulator, use an Intel System Image and install the HAXM device driver

I have downloaded full package for 21 and now which image i should install and what is AVD its asking for