The CMSAppBase.cs file was present in Kentico 7 and previous version. It was used to manage the application events. This file was removed in Kentico 8 and later. It was a system file not designed for customization. It looks like it was customized and now your custom code is throwing errors since this file was removed.
You need to find the file in the older version and check what events were handled in the code and then rewrite this code to the version 8 format, using the custom event handlers: https://docs.kentico.com/display/K8/Handling+global+events and https://docs.kentico.com/display/K8/Reference+-+Global+system+events