To expand further on Rilgamon's post, slash commands pass everything after the actual command into only the first variable. arg1 is nil here.
In your function,
Lua Code:
func == "add <Linen Cloth>"
and
which means you are doing
which causes the error.
If you want add/remove commands, you have to "take apart" the first variable. One common method is
Lua Code:
SlashCmdList.SERVANT = function(input)
local command, link = strsplit(" ", input)
where
and