Automation emails do not support that tag. They support only a tiny handful of html tags. You can try using CSS to see if you can get a line that way, but if it doesn’t work, then it doesn’t work. I suspect that markdown for a horizontal rule also does not work.
This support article lists the tags near the end of the page in the FAQ.
How about Markdown? Even the Scripting app can output stuff like this:
A triple underscore nets you one break. Ditto for the asterisk and the regular dash.
I’m not well-versed in Airtable email integrations but I’m fairly certain they have the Scripting app on a way harder security lockdown than a mail-merging software might require. And Markdown is a superscript of HTML, so I’d definitely start there.