Why I cant download fb apps at appstore using mobile data or wifi

Okay, Facebook has offices in Malaysia. I don't know what MY set is but I am assuming it is a retailer / rental business.

If the phone was new at the point of purchase and not bought in from a country where Facebook is banned, the app should be free in the app store for download..

If you mean Facebook apps, in some middle Eastern countries you are not allowed to use Facebook by government decree.

What country are you resident in and which country was the iPhone first purchased from.