bios:338: [string "startup"]:5: 'then' expected

[then expected]

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rednet.open("right")

while true do
local,id,message,dist = rednet.receive()
if message = "des" then
for i=1, 12 do
redstone.setOutput("left" . true)
sleep(0.4)
redstone.setOutput("left".false)
sleep(0.4)
end
end
end

This exact issue is covered in one of the sticky posts.