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:
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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?
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I work on that later.