Equipment modification screen.
At first glance, modifying weapons and/or armor in Mass Effect Andromeda may seem relatively simple, however, there are a few rules. To even modify your gear by putting special mod items in it, you first need to find a weapon and/or armor with the appropriate number of sockets. Each weapon (except melee) has 2 sockets. Unfortunately, the armor pieces are completely different – only the torso armor has one slot for the unique Fusion Mod.
When you have purchased the desired weapon and/or armor piece and mod, you just need to enter the equipment screen (it appears every time you leave the Tempest ship, it can also be accessed from Tempest and Forward), select the weapon/armor and press the button shown on the screen to open the mods screen. You must then accept your choice (and add more mods if you wish) and leave the loading screen. Don’t forget to equip the weapon you just modified!
There is one limitation related to item mods – you cannot install two mods of the same type on the same weapon/armor piece. For example, you cannot mount two scopes on a sniper rifle. This places some restrictions on the player, but given the number of different weapon/armor mods available in the game, you shouldn’t have any trouble finding something that suits you.
Note. The number of sockets available to an item can be increased with add-ons, special items that can be placed on an item when it is created.
Source: https://guides.gamepressure.com