When Upgrading Vizit, it is possible to receive the following error message:
The site with the id SITE_COLLECTION_GUID_HERE could not be found.
If a Site Collection was deleted between the last time the "Gradual Site Delete" Timer Job was run and the moment the Vizit installer is run, this error could appear. Vizit queries SharePoint for any of its features that may require upgrade. If sites have recently been deleted, this error will be thrown.
From the Job Definitions page in Central Administration, locate the "Gradual Site Delete" Timer Jobs for each of your Web Applications and run them. Vizit will not successfully upgrade until this is completed.