While the version v1.5.1 for Unreal 4.18 and 4.20 went online two days ago, as reported in the release thread, there was a technical issue on the Marketplace backend with the same version for Unreal 4.19. The issue has been solved a few hours ago and that version is now online, too.
I don’t know if that answers your problem, though. What is your situation and what are the operations that you are performing? In order to upgrade the plugin you typically do not click on “Install to Engine” but on the “Installed plugins” right below the engine instance.