TBH, I'd be surprised if that's your issue, but I'll go into that a little detail on my thoughts further down.

Regardless, there isn't an easy (or recommended) way to install PHP 7.3.16 that I am aware of.

To give you some context, TurnKey is based on Debian. The way that things work in Debian (and other "fixed" or "point" type Linux distributions) is that at initial stable release time, a particular version of software is "frozen". In the case of PHP in Debian 10/Buster (the basis of TurnKey v16.x) that is PHP v7.3.14.

Any future security issues are resolved by specially crafted backported security patches. These patches contain just the minimal set of changes to resolve the security issue. This provides an incredibly stable environment, with minimal maintenance required, but without sacrificing security; ideal for a headless server.

If you're hell bent on changing the PHP version (not recommended unless you have a confirmed need) there is one way that I can recommend. One of the Debian (and Ubuntu) PHP package maintainers; Ondřej Surý, provides his own PHP repositories for newer (and older) versions of PHP. However, he is packaging PHP direct from upstream (i.e. direct from PHP themselves) so only ever provides the latest version. I.e. he doesn't do security patches, he just repackages the latest version (which will include any security fixes). So if you wanted to use that, you'll need to upgrade to PHP 7.3.19. And it will mean that to keep PHP on your server secure, you'll need to manually apply updates yourself regularly. TurnKey installs Debian security updates automatically every night.

Like I said initially though, I'm highly sceptical that that is your issue. In case you missed it, I'll repost a modified/updated copy of what I posted before on your earlier post (which was really broken and I'm still not sure why?!).

You can find lots of sites with troubleshooting info and steps for the fairly common "WordPress white screen of death". WordPress themselves note it and other common errors with some troubleshooting and fix suggestions.

It sounds like you were trying to enable debugging before, which is a good step, but the issue almost certainly isn't anything to do with a warning (warnings are not errors). Note that the above WordPress page links to this WordPress doc page which looks like it might be a handy reference.

If you haven't already, I'd recommend disabling all of your plugins and going back to the default theme. Hopefully that should get your site going again. Then you can enable your plugins and themes, one by one until it breaks. Then you can discuss with the plugin/theme developer what requirement they have and make sure that you are matching them on your server.

If you can't do any of that on TurnKey (because it's not working) then it might be worth making those changes on your other server before taking a backup (I assume that's how you transferring your WP site?).

If you want more of a hand, please be a bit more explicit about exactly what you've done and how. As well as what you've tried to fix it and what the results of that were.