First, I should explain how Blue-on-Blue Damage works in this Game. There are two Ways you can cause Damages on your Teammates, one is direct Hit like spitting your Bullets on Ellis' Face, and the other is indirect like Molotov, Gascan or Barrel that can lead to widespread Fire. So, in order to avoid terrible missing or against trolling by Troublemakers, you have to disable both of them. Here I will demonstrate how to do that:
Step 1 is checking your current Difficulty; for Instance, I am playing on Advanced Level, in this Case I shall refer to the Convars that have -Hard as Suffix under Multiplayer Panel.
Step 2 is once locating the aforementioned, set both Friendly Fire and Flame Damage to outright Left, which means disable, and that's it.