Joomla Site Plugin works successfully ONLY on PHP 7.2 or lower!
For PHP 7.3 Joomla core team has to add this feature I guess.
For temporary solution on PHP 7.3+ you can check our post here
https://joomla.stackexchange.com/a/28736/8936
Plugin works on Joomla 2.5/3.x.
We don't provide support to this plugin.