Problems with scihub itself are common, although often only of short duration. Or scihub itself may be blocked in your location. You should go first to the actual scihub site (the particular scihub mirror URL that your plugin is set to use) and see if you can download the PDF in question there.

You can check the scihub reddit to see if there are systemic issues being reported by others, and also get other mirror URLs to try. There are sometimes captchas associated with scihub downloads, which plugins are not always able to handle. If this is a persistent problem with the plugin despite being able to use the scihub site directly, you need to ask the plugin developer (ie at their github site).