The Line Racks Schematic for the Line Gun in Dead Space Remake can be found in the Dr. B Warwick - CPO office. You find this area during the Chapter 5 - Lethal Devotion main quest.
Line Rack Schematic Location
As you make your way through Chapter 5 - Lethal Devotion in the Dead Space Remake, you should find yourself on Floor 5 of Medical, as you’re looking for the source of the broadcast for your main objective. In the northwestern part of this floor can be found the Dr. B Warwick - CPO office, which requires at least security clearance level 2 to enter. You get this level of security clearance towards the end of Chapter 4 - Obliteration Imminent, so you should be able to open the door to the office without any problems. However, if for any reason you don’t have it, you can always return here later.
Once inside, you’ll see the Line Rack schematic on the table in front of you. You should have at least one empty slot in your inventory to take it, but if your inventory is full you can always dispose of some expendable items such as oxygen tanks, med packs, or obsolete ammo for guns that you aren’t using. Now that you have the schematic in your inventory, you can visit any store to “learn” it and begin purchasing Line Racks, which are the primary ammunition source for the Line Gun. They cost 2,100 Credits per rack.
This is one of the easiest schematics to obtain, but it’s also frequently missed, so don’t feel too bad if you need to come back! It’s a good idea to keep making manual saves as you progress through the game to give yourself as many opportunities to return to old areas before you’re locked out from them, but in the worst case scenario you can always get this schematic during a New Game Plus playthrough.