Hi, k9kate.
Welcome to the Apple Support Communities. We’re glad you decided to reach out about sending messages on your new MacBook Pro. If we understand correctly, you’ve already tried most of the steps in this article:
If you can’t send or receive messages on your Mac - Apple Support
Are you trying to send an iMessage or SMS/MMS messages with your MacBook? If you’re trying to send an iMessage check to make sure you’re signed in with your Apple ID and see if you have an activation error or issue.
Use Messages with your Mac - Apple Support
If you’re experiencing issues with your Apple ID in iMessages, the remaining steps in this article may help:
If you can't sign in to your iMessage account or FaceTime on your Mac - Apple Support
Once you have iMessage set up, you can send and receive SMS/MMS messages, too. This article walks you through that setup:
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
If sending these types of messages is your issue, pay particular attention to #3. It may just be an issue where you need to go in and choose your new MacBook Pro to send/receive messages in Settings on your iPhone.
All our best.