Fatal error: Uncaught Error: Interface "Illuminate\Contracts\Validation\Rule" not found in /var/www/html/lib/pkp/classes/invitation/invitations/userRoleAssignment/rules/UserGroupExistsRule.php:20 Stack trace: #0 {main} thrown in /var/www/html/lib/pkp/classes/invitation/invitations/userRoleAssignment/rules/UserGroupExistsRule.php on line 20