A weakaura custom trigger requires a function object to work. You're getting an error because C_Timer.After is a function call in this scenario. Wrap what you have in a function object:
Lua Code:
function(event)
C_Timer.After(3, function()
if event == "BOSS_KILL" then
return true
end
end
end
If you're confused, basically weakaura trigger custom code has to start with the word "function" and end with "end", no exceptions, unless you have a global function to use, then you only use the function name and nothing else.