My Advanced Installer version is 18.6.
We use Pre-install prerequisites for Application. Is it possible to have the Repair option install any prerequisites that are missing (were deleted by user or system)?
Unfortunately, that is not possible for pre-install prerequisites.
Basically, the "Repair" operation checks for whether a feature/component is missing and tries to install it if it is.
Pre-install prerequisites are embedded in the EXE bootstrapper and launched before the main MSI is launched.
To work this around, you can add the prerequisite as "feature-based". This way, a feature will be created for that prerequisite in the "Organization" page. Upon repair, if that feature is missing (e.g. the user uninstalled the prerequisite), the setup will try to reinstall it.
Depending on your prerequisites, this should be pretty easy to achieve (just moving the prerequisite from pre-install to feature-based). However, there are some prerequisites which can not be installed as "feature-based" (such as .NET Framework, some versions of SQL Server, etc.). This is simply a limitation on how the respective setup was designed.