I've run into something interesting (just installed nop 3.9).
If I browse locally on my dev machine to http://localdev/nopcommerceSite/, the nivo slider on the main page shows. However if I go to http://localdev/nopcommerceSite it doesn't
I understand that we enter the "url" for our store in admin, but in real life, it's totally plausible that someone leaves out the trailing slash. Like when I just came to https://www.nopcommerce.com
As it's on my local machine there isn't a ".com" might that be an issue? Nivo plugin expecting the domain suffix?
As per your query, you can check in Store URL in Administration area on Store Edit Page. If your Store URL is not ending with "/" than please add symbol i.e. "/" in the end of Store URL name.