Your PHP Version is 7.4.3-4ubuntu2.28
The correct include statement to include Form.php is the following:
include_once rtrim($_SERVER['DOCUMENT_ROOT'], DIRECTORY_SEPARATOR) . '/phpformbuilder/Form.php';
This is the default path used in all templates. You've got nothing to change.
/var/www/html/phpformbuilder/plugins/
/phpformbuilder/server.php
/var/www/html/phpformbuilder/server.php
/var/www/html
/var/www/html
/var/www/html/phpformbuilder
/var/www/html
http://161.35.59.153/phpformbuilder/plugins/