Okay he didn't cast a spell but he did seek to impress her but she dismissed him. He doesn't elaborate on what really happened. He lost her favor somehow. The question is why. We will have to wait for more content to find out what happened.
From how he phrases it, Mystra neither dismissed him nor started ignoring him for any particular reason. She simply got bored of her human pastime, he tried to win her back by messing with Netherese magic, fucked up, and she kept ignoring him. Very greek of her, but what god isn't fickle.
