Error message: 550-Please turn on SMTP Authentication in your mail client

Ah, the displays got shuffled around.

Select Server Settings, and select the Outgoing Mail Account selector to Edit SMTP Server List.

That'll get you to the SMTP Server settings, and which are some of the more well-hidden settings on macOS.

Select the SMTP Server, and fill in the settings. SMTP with Password authentication or MD5 authentication would be typical. For some background here, Apple has available some documentation for manually-configured settings as does Google Gmail, which is what this path is. (It's also possible to use automatic settings for iCloud and Google Gmail, via Internet Accounts preferences..)

And the Advanced will look like this:

Mail.app > Mail > Preferences

Select Accounts > Account Info

Select the Outgoing Mail Server (SMTP) field and pick Edit Server Lisr from the pop-up.

Select ine of he SMTP Servers that’s giving you trouble

Select Advanced

Select Password Authentication, and use TCP Port 587, and select SSL/TLS connection security

Enter the username and password for the SMTP Server

Select Test Account Settings

Exit snd Save your way out.

Try sending your mail again.