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It is just a cache.
After buying premium version of the plugin just uninstall free version of WOOF, download from https://codecanyon.net/downloads zip with WOOF and unpack it.
Inside of the unpacked zip you will find woocommerce-products-filter.zip - Install it on your site. Then after installation go to the plugin settings page and press CTRL+F5 or CTRL+R- and your browser cache should be cleared ...