Before you can use Validated Forms you will need to run a maintenance script that will search all your pages in the wiki for FlexForms. That list will then show up on FlexForms Special page.
to run the maintenance script: in MediaWiki 1.39.13 or higher
From the root folder ( where your wiki is installed )
php maintenance/run.php ./extensions/FlexForm/maintenance/syncPagesWithForms.php
in MediaWiki 1.39.12 or lower
From the root folder ( where your wiki is installed )
php extensions/FlexForm/maintenance/syncPagesWithForms.php
