iMessage won't work on my MacBook Pro

Send a message

1. Open Messages. You might be asked to sign in with your Apple ID. Can't sign in?
2. Click the New Message button  at the top of the Messages window.
3. Type the name, email address, or phone number of one or more recipients. Or click the Add button  and choose from your contacts.
4. Press Tab or click the message field at the bottom of the window, then type your message.

• • To include emoji, click the Emoji Picker button .
  • To include an image or other file, drag or paste it into the message field.
  • To include an audio recording, click the Record Audio button  and record your message.
  • To include other items, such as Memoji stickers, GIFs, and message effects such as balloons or confetti, click the Apps button . This feature is available in macOS Big Sur or later. 
  • To help bring your message to the attention of a recipient, include @ before their name, like @John. Or click their name after you type it. Their name is then highlighted in the message, and they will receive a notification letting them know that they have been mentioned. This feature is available in macOS Big Sur or later.

You can also refer to this apple blog Use Messages on your Mac - Apple Support