You can only extend to a line and not a node. This is intentional - it’s a bit complicated to explain but basically allowing nodes to be extended to caused more problems then it was worth. You could, for example, have multiple nodes at the same point in which case it’s impossible to tell what node to extend and connect to. Also, when I originally tried to implement node-to-node extending it caused all sorts of weird bugs that were due to the math required to detect where I should be extending to.
It’s on my list to take another pass at, but for now trim and extend only works when the thing you are extending to or trimming on is the body of the line and not one of the nodes.