@true,
That means there is a syntax error in start.lua (the scene file).
You see this error when trying to load a composer scene which has one or more syntax errors. Unfortunately, you may lose some of the error information when loading via composer.gotoScene(), but a scene file is effectively a module, so you might be able to find the error by doing this:
require "start" --composer.gotoScene( "start" )
This will load the scene file via the Lua require() mechanism. It may help you find issues that are otherwise hidden by the scene loading process.
Once you fix the errors, you can comment that out and use gotoScene() to load your scene.