Module script argument 1 missing or nil help?

The first thing I notice is the fact that you are calling the function as soon as you get it from the module script without any arguments. This is most likely the reason you are getting the error. Do something like this instead:

AddCoinFunc = require(game.ServerScriptService.AddCoins)
AddCoinFunc('Trolna' , 100)

Another thing I noticed is the fact that you are using the player name. This is very dangerous and should never be used because players have the ability to change their name. Use their player ID instead which is a constant unique value attributed to every player. Finally, your call to the function should be wrapped in a pcall() function in order to catch any error Data stores always have the possibility of error because it has to make web calls to the Roblox servers. Your final scripts would look like this:

Module Script:

CoinData = game:GetService('DataStoreService'):GetDataStore('Coins')
function AddCoins(plrID, toadd)
    print(plrID)
    if CoinData:GetAsync(plrID) then
        CoinData:IncrecementAsync(plrID, toadd)
        print(CoinData:GetAsync(plrID))
    end
end
return AddCoins

Script:

AddCoinFunc = require(game.ServerScriptService.AddCoins)

local success = pcall(function () return AddCoinFunc(Trolna.PlayerID , 100) end)
if not success then
    --handle case where data store errors
end

Module script:

local module={}
CoinData = game:GetService('DataStoreService'):GetDataStore('Coins')
function module.AddCoins(plr, toadd)
    print(plr)
    if CoinData:GetAsync(plr) then
        CoinData:IncrecementAsync(plr, toadd)
        print(CoinData:GetAsync(plr))
    end
end
return module

Other script:

a = require(game.ServerScriptService.AddCoins).AddCoins('Trolna', 100)