You sent a message to UI but didn't receive it because you don't have an event handler. You need to add this to ui.html:

onmessage = (event) => {
  console.log('Message from code.js', event.data.pluginMessage)
};

Please learn how to send and receive messages from/to code.js and ui.html from this article: https://www.figma.com/plugin-docs/creating-ui/#sending-a-message-from-the-plugin-code-to-the-ui