当我运行以下命令时：
ionic build android
在我的Windows 7计算机上的命令提示符中，弹出以下错误：
BUILD FAILED
C:\adt-bundle-windows-x86_64-20140702\sdk\tools\ant\build.xml:542: Unable to resolve project target 'android-21'
Total time: 1 second
C:\Users\<user name>\Hello_Android\platforms\android\cordova\node_modules\q\q.js:126
throw e;
Error code 1 for command: cmd with args: /s /c "ant-debug -f "C:\Users\<user name>\Hello_Android\platforms\android\build.xml" -Dout.dir=ant-build -Dgen.absolute.dir=ant-gen"
ERROR building one of the platforms: Error: cmd: Command failed with exit code 1
You may not have the required environment or OS to build the project
Error: cmd: Command failed with exit code 1
at ChildProcess.whenDone <C:\Users\<user name>\AppData\Roaming\npm\node_modules\cordova\node_modules\cordova-lib\src\cordova\superspawn.js:131:23>
at ChildProcess.emit <events.js:110:17>
at maybeClose <child_process.js:1015:16>
at Process.ChildProcess._handle.onexit (child_process.js:1087:5)
有人可以帮我解决这个问题吗？