how is this wrong?

I have a code that runs text across the screen but for some reason it is saying on line 11 "attempt to index? (a nil value)
Here is the code :
local mon = peripheral.wrap("up")
local pos = 10

while true do

if pos ==-36 then

pos = 10
end

mon.write("Paradox Nuclear Reactor Facility")
pos = pos-1
end

The correct side name to use would be "top", not "up".

ok thanks

Depending on which version of CC you're using you could also use the peripheral function that automatically finds and wraps the peripheral

local mon = peripheral.find( "monitor" )