Argument Error, don't know what it means, any help?

Consider sanity checking your inputs.

local create = script.Parent
local frame = script.Parent.Parent
local id = script.Parent.Parent.groupID.Text
local plr = game.Players.LocalPlayer

local ap = require(game.Workspace.TrelloAPI)


create.MouseButton1Click:connect(function()    
    if tonumber(id) and plr:GetRankInGroup(tonumber(id)) == 255 then --Line 12
        wait()
        local group = game:GetService("GroupService"):GetGroupInfoAsync(tonumber(id.Text))
        local BoardID = ap:GetBoardID("Appalachian Security Contract Application")
        local ListID = ap:GetListID("Applied",BoardID)
        local CardID = ap:AddCard("[NS] " .. group.Name, game.Players.LocalPlayer.Name, id, ListID)
        print("He's the owner!!!!")
        script.Parent.Parent.create.Visible = false
        wait(3)
    game.Players.LocalPlayer:Kick("Thank you for choosing Appalachian Private Security Company, we hope you choose us again! -APSC")

    else
        print("He's NOT the owner!!!!") -- Or, the ID is invalid
        script.Parent.Parent.Warn.Visible = true
        script.Parent.Parent.create.Visible = false
        script.Parent.Parent.groupID.Text = "Sorry, you MUST be the owner!"
        script.Parent.Parent.groupID.TextColor3 = script.Parent.Parent.groupID.TextColor3(255, 0, 0)
        wait(5)
        script.Parent.Parent.groupID.Text = "Enter group ID here"
        script.Parent.Parent.groupID.TextColor3 = script.Parent.Parent.groupID.TextColor3(0, 0, 0)
    end
end)

What's new?
On line 12, we now check to see if the input is actually in number form by seeing if tonumber() returns a number or nil. If the input is not in a valid format, tonumber returns nil, which must be captured when handling user input.