In the file:
/etc/apt/sources.list.d/nginx.list

I changed:
deb https://nginx.org/packages/mainline/ubuntu tricia nginx
to:
deb https://nginx.org/packages/mainline/ubuntu bionic nginx

Then the "apt-get update" worked.

However, I used the Mint Software Manager to install Nginx, but the progress indicator stalled at about 80%. I closed the Manager acknowledging the message that a task was still running.

Of course, that probably caused another problem:
-----
E: Could not get lock /var/lib/dpkg/lock-frontend - open (11: Resource temporarily unavailable)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
-----
I work on that later.