That FindByName method on the releases repository doesn’t actually have a functionality. It only inherited it from one of the common resource interfaces.
What you should be using is $Repository.Projects.GetReleaseByVersio($project,"1.0.0") where $Project is an Octopus.Client.Model.ProjectResource object.
If you are still not using the open source project Octoposh, I highly recommend you to do so. In this case you could use Get-OctopusRelease -ProjectName "MyProject" -ReleaseVersion "0.5.0"