tpt.set_property(...) isn't a function. tpt.set_property is a function, but you can't pass it directly to tpt.register_step while specifying what arguments it should get. Instead, create a new function that calls tpt.set_property with the right arguments, and pass that to tpt.register_step.

In Lua, you can create a new function using:

function()
 --code goes here
end

So in this case, you would use:

tpt.register_step(function() tpt.set_property("type","none","sing") end)