I keep getting "Argument 1 missing or nil"?

If we read the error message carefully, we'll see that the game is complaining because the first value given to the function at line 16 is nil. In other words your variable plr is nil. plr becomes nil because you are trying to get the localplayer in normal script.

If i understand your code correctly, you want to check if the player who touched the badge owns a specific gamepass. If that's the case, then you don't want to use the plr variable at all, instead use the already declared variable player (at line 8) to get the user id.

-- replace line 16 with this instead:
local userId = game.Players[player.Name].userId
if game:GetService("MarketplaceService"):PlayerOwnsAsset(userId, ID) then

local Part = script.Parent
local plr = game.Players.LocalPlayer
local ID = 7697627

Part.Touched:Connect(function(HIT)
    local H = HIT.Parent:FindFirstChild("Humanoid") 
    if H then
        local player = game.Players:GetPlayerFromCharacter(HIT.Parent)  
if player then
    local leaderstats = player:WaitForChild("leaderstats")
    local Currency = leaderstats.Coins
    local Selling = leaderstats.Clicks
        if Selling.Value > 0 then
            Currency.Value = Currency.Value + Selling.Value *1
            Selling.Value = 0
            if game:GetService("MarketplaceService"):PlayerOwnsAsset(plr.userId, ID) then 
            Currency.Value = Currency.Value + Selling.Value *2
            script.Parent.Coins:Play()
        end
      end
    end
  end
end)

Lol you see people struggle with this all the time, it's just strange that marketplace service requires a userId instead of a player object so it confuses people :)