When the install script for Wallabag don't find Tidy (php fonction), it cease the installation process. When you install Wallabag manually, it only serves you a message and continue the installation. Without Tidy Wallabag will work with few problems.
Is it possible to mimick the original installation of Wallabag in Softaculous?
Since Wallabag allows installation without Tidy we have updated the package and removed the requirement check for Tidy.
Please ask your host to execute the CRON command on your server to get the latest package :
http://www.softaculous.com/docs/Immediate_update