At the end of the activation it says GTA IV FATAL ERROR DD3D50.WHAT CAUSES THAT

Hello

DD3D50 - Direct x 9 video card required. you are most likely trying to play the game on a laptop or on a old desktop computer, a newer computer with a video card with hardware direct x 9 acceleration is required.

This is not a fault, it is a requirement of the game in order to work, always double check the requirements that read on the box before purchasing a game.