Help - I'm not really in a position to roll PHP back to an older version??
This is current Debian stable - https://packages.debian.org/stable/php-fpm - php 7.3
But maybe above answer is not correct: install_dev/install_version.php says:
define('_PS_INSTALL_MINIMUM_PHP_VERSION_', '7.1.3'); define('_PS_INSTALL_MAXIMUM_PHP_VERSION_', '7.5.0');