Not sure if anyone’s reported this yet, but the Substance Plugin seems to be bugged in 4.26
On the marketplace it’s says it’s supported, but it isn’t possible to install it to the engine, and Unreal will just say “You cannot install this plugin as it is already installed on all compatible engines”. It doesn’t show up if you check the plugins list in the editor either, and doesn’t show up in the launcher’s list of installed plugins.