hactool --keyset=Z:\path\to\prod.keys [other options]
The most common reason is that you have not dumped your own prod.keys from your Nintendo Switch. Unlike a simple configuration file, you cannot download prod.keys legally from the internet because they are copyrighted and console-specific. hactool prodkeys does not exist fixed
Open prod.keys in a text editor. It must contain lines like: hactool --keyset=Z:\path\to\prod