Hello Mac_Donn,
We understand you aren't able to send outgoing email messages on your Mac. It appears you received a very specific error. That error would indicate an issue with your outgoing mail server. Since you weren't able to choose anything other than iCloud, you will need to go in to your Mail preferences to make adjustments to your mail account outgoing server.
The information you'll need to correct will depend upon which mail provider you are using. You can start with this helpful article for troubles with sending email: If you can’t send or receive email on your Mac - Apple Support
That article includes steps to remove then re add your email account. That may resolve the issue. You can also use this guide to change Server Settings in Mail on your Mac: Change Server Settings in Mail on Mac - Apple Support
You'll want to check with your mail provider if you are unsure what the outgoing server information specifically is. Reply back with any questions.
Best regards.