You must complete The Prologue missions so your character is allowed to freely roam Los Santos. Play in GTA V in Story Mode and complete all missions of the Prologue. The prologue contains four missions: The Robbery, The Threat-Assessment, The Escape, The Getaway. There are several tutorials on the internet and on YouTube about how to finish the missions.
If you don’t want to play the game at all, the mod installer leaves a save game file for the 100% completed game. Find more details under Do I have to play the game to use GT Bike V?