Adding to this:
Apache on older Ubuntu versions (as 16.04 certainly is) tends to use a Multiprocessing Module named "Prefork" which comes with it's own PHP implementation embedded into the Apache process. To update this, execute the following:
"sudo apt-get install libapache2-mod-php7.2" (or whichever version you prefer)
"sudo apt-get install libapache2-mod-php7.2" (or whichever version you prefer)
You would also be well advised to switch from Prefork to Event (with php-fpm) if you're comfortable doing that.
Downgrading from PHP7.4 should also be a priority, since the cronjob will use your CLI-version of PHP.
After updating your PHP version, ensure that the required PHP extensions for Moodle are still installed.