Mine is fixed, but not sure what I did. I am now using mail.app to send invoices from the mail button above the invoice.

It may be that I had quickbooks 2016 and 2019 on my computer before I downloaded 2020, because it started working after I deleted 2016, 2019, 2020, and then re-loaded 2020 from Intuit.

I am sure I went to Mac, system preferences, security, automation, and the quickbooks option was still not there.  Then, I went to quickbooks email preferences and chose "default email application" and it started working!

Then, I went back to Mac system preferences, security, automation, and the "quickbooks icon, and mail app" checkbox" was there, and it was checked... So, maybe reloading without the old QB on my Mac solved it, and checking default mail app in QB email preferences drove the option back into the Mac automation?

I had also tried using a "custom" email account in QB using an "app password" they provided, and then  turning off 2 step authentication in gmail, which did not work, and google did not like, so I turned it back on, and mail app still works.