First, don't use "Trigger once" like that. Try not to use it at all, until you have solid understanding of how events work in Construct. I've seen too many projects where "Trigger once" was used wrong and was causing some nasty bugs.
If you want some actions to be executed once, use triggered events, like "On start of layout" for example.
Drawing Canvas is an advanced feature and it's not required in most games. There are easier ways to draw a line, you can use a Sprite or Tiled Background object.
If you are sure you want to draw on Drawing Canvas, you need to add it to your layout. Looks like you've added Drawing Canvas object to the project, but there are no instances of it on the layout.