Weapon upgrades

I’m new to the game (level 6) and I’ve seen people putting attachments on their weapons and levelling them up, but when I go to the armoury all I can do is look at my weapon stats and swap weapons, there’s no options for customisation and no attachments or levels.

I’ve played a few missions already and haven’t seen anything pop up saying my weapon has levelled up, is there some DLC you have to buy for it? I have all the most recent updates installed so I’m not sure what I’m missing. Everything I’ve seen online just says go to the armoury and spend req currency for attachments but there’s nothing in my armoury to purchase.

Any advice would be appreciated.