This mission is only available if you selected the Loud approach to the Jewel Store Job heist in the mission Casing the Jewel Store. This is the only heist setup mission for the Loud approach.
Michael receives a text from Lester. To get the weapons for the heist Michael must ambush an LSPD tactical squad truck. Michael intercepts the truck, the NOOSE guards retaliate. After killing the tactical officers and evading the cops, Michael then delivers the vehicle to the garment factory in front of the Darnell Bros. Lester then tells Michael that they are ready to go for the heist.