The note says: "WHERE IS MY ASSHOLE FOR MY BULLYING for friendship with him?" you see Flowey, who has apparently forgotten about that damn note and is learning to conjure. Yea, the very thing you lack.
A third time he hears the voice call out: “Daaave.” Now, his heart pounding, unable to see anyone calling out, he explodes in anxiety and frustration and shouts out, “My name is not Dave!”