"Cannot send message using the server iCloud"

Here are instructions, it's too much detail to insert in a forum discussion reply but it explains how to proceed:

https://support.apple.com/guide/mail/add-or-remove-email-accounts-mail35803/mac

Your GMAIl seems to be misconfigured was it is trying to use a non-GMAIL server to send GMAIL. One way to address this is to go to Internet Accounts (System Preferences) and remove the GMAIL account and then recreate it, letting Google automatically configure it. I suggest making one or two complete backups of your computer before doing this in case you do not like the results and want to restore back to the way things were.