When you install Notepad++ (latest version 7.5.1), the plugin manager isn’t automatically included anymore. That’s because many Notepad++ users didn’t like the sponsored commercials in the latest versions of plugin manager. So, you will have to download the plugin manager manually and add the downloaded files to your Notepad++ install directory.
You can download the plugin manager for Notepad++ version 7.5.1 via the following Link (32-bit or 64-bit)
Next copy the extracted files to your notepad++ plugins and updater folder.