@ubaldo-p ,
Apparently no one else has that error (at least none of the 63-or-so views of your post have resulted in anyone willing to admit to the problem). I have used the Compare plugin successfully with v8.1.9 without difficulty.
But I didn’t have to install it, because Compare plugin already was installed when I upgraded to v8.1.9.
So today, since you hadn’t gotten a response, I took a fresh Notepad++ v8.1.9 portable unzip, and used Plugins Admin to install the plugin, and there was no difficulty.
The things I would check if I were you:
make sure you have write permission in the C:\Program Files\Notepad++\plugins directory (or the equivalent for your installation) – or you could temporarily run Notepad++ as Admin in order to install the plugin, if your normal user doesn’t have necessary privileges.
make sure you have the proxy settings (
?-menu,
Set Updater Proxy) correctly defined in Notepad++ if your machine requires a proxy for internet connections.