Able to receive Gmail but unable to send on Mac Pro, OS 10.13.6

I am switching to Gmail for a variety of reasons. I have set up Gmail on my MacBook Pro and it is working perfectly. I am having an issue on my older Mac Pro, running OS 10.13.6.

MacMail is Version 11.5 (3445.9.7)

I have what looks like identical accounts on both computers but I am only able to receive Gmail on my Mac Pro, I cannot send it.

While attempting to add a new Mail account via the Internet Accounts under my System Prefs, I get a message stating: "Unable to verify account name or password." And that is as far as i t goes.

I am receiving any and all email sent TO my gmail account, it just will not send it from this computer.

I am stymied.

I am using the correct user name, the correct email address, the correct password. Just not sure what to try next. Any help would be greatly appreciated.