Method has not yet been implemented - Error with installApp method on appium 1.5.3

Appium version 1.5.3
Java_client 4.0.0
Xcode : 7.3.1
Simulator: IPhone 6 s

Hi,

I am trying to use these methods with above versions mentioned

driver.isAppInstalled(“xyz”);
driver.installApp(“x.app”);

But getting Error - ‘Method Not yet implemented’
App is in native context.

Any suggestions on what could be happening here.

Thanks in advance!