@jjcarolan,
This sounds very much like your server is *not* on PHP 7.3
If you’re seeing this when you’re trying to install LifterLMS that means WordPress itself is identifying that your server doesn’t meet our minimum requirements (PHP 7.2 — see more here: https://lifterlms.com/docs/minimum-system-requirements-lifterlms/)
If this is what’s happening then your host is incorrect and your not actually using 7.3.
You can double check what’s actually running on your server with the new site health check info report (available in WP core 5.2 or later) by navigating to Tools -> Site Health -> Info and then opening the “server” section.
Can you help orient me to the error your seeing by posting a screencap and also posting a copy of your WordPress site health check report information (click “Copy site info to clipboard”) on the site health screen I mention above and post that to a service like pastebin and paste that link here for me to review.
Thanks