Adding coins to a leaderstat when touching a brick?

You almost got it right! But there is 1-2 problems stopping you from completing your ask, let me help you out.

local function onTouched(part)
    if part.Parent:FindFirstChildOf('Humanoid') then -- first check if it's a potential player
        local player = game.Players:GetPlayerFromCharacter(part.Parent) -- see if its a player
        if player then -- final check

            if Player:FindFirstChild('leaderstats') == nil then -- check if player already has leaderstats or not
            local leaderstats = Instance.new('Folder', player) -- you forgot to parent it
            leaderstats.Name = 'leaderstats'
            local Coins = Instance.new('IntValue', leaderstats)
            Coins.Name = "Coins"
            Coins.Value = 0 -- default value at start
            end

            if player:FindFirstChild('leaderstats') ~= nil then -- check if player now has leaderstats      player.leaderstats.Coins.Value = player.leaderstats.Coins.Value + 200 -- give them 200 coins

            end


        end
    end
end

Though ABritishChap's answer is correct, theres a way to make it simpler.

Heres how:

local function onTouched(part)
    local plr = game.Players:GetPlayerFromCharacter(part.Parent)
    if plr then -- If its a player
        if not plr:FindFirstChild("leaderstat") then -- Check if player already has leaderstats or not
            local leaderstats = Instance.new("Folder", plr) -- Create leaderstats then parent it to the player
            leaderstats.Name = "leaderstats"
            local Coins = Instance.new("IntValue", leaderstats)
            Coins.Name = "Coins"
            Coins.Value = 0 -- Default value at start
        end
        if plr:FindFirstChild("leaderstats") ~= nil then -- Check if player now has leaderstats      
            plr.leaderstats.Coins.Value = plr.leaderstats.Coins.Value + 200 -- Give them 200 coins
        end
    end
end