Unfortunately in 2.6 the tooling for generating reports is not fantastic. If you are familiar with Raven Studio you could query the database directly.
I think the easiest method would be to migrate your data to the latest version of Octopus which would make generating the deployment audit extremely easy. The licensing only prevents deployments so you could keep your 2.6 installation and take a backup to import into the latest version and use the much improved Audit screen, or reports, or do custom SQL queries.