Math.Random and string won't work together?

Well if you print this out

local Strings = {'A', 'B', 'T', 'U'}
local Boss = Strings math.random(1, #Strings)

you will get a table so what you want to do is

local Strings = {'A', 'B', 'T', 'U'}
local Boss = Strings[math.random(1, #Strings)]

Even if that is how you do it your script is all wrong it should say

local ReplicatedStorage = game.ServerStorage
local A = ReplicatedStorage:WaitForChild('Asher')
local B = ReplicatedStorage:WaitForChild('Banana')
local T = ReplicatedStorage:WaitForChild('Trump')
local U = ReplicatedStorage:WaitForChild('Unicorn')
local DropperPart = workspace:WaitForChild('DropperPart') 


local Strings = {A, B, T, U}
local Boss = Strings[math.random(1, #Strings)]

while wait(math.random(8,18)) do
    local m = Boss:Clone()  -- this line
m.Parent = DropperPart

game:GetService('Debris'):AddItem(Boss,300)
end