Woocommerce custom options could not detect an active Woocommerce plugin

I get this message when I activate this plugin on my website. But the Woocommerce plugin is definitely active. I’ve tried turning off both and reactivating them each, WC first. To no avail. Any insight as to how Custom Options determines that the installation exists or not, so I can troubleshoot further, would be greatly appreciated.

This is a pretty standard WPMU setup, the site in question being on the primary domain and main WP site. I don’t see any error messages at all in my log files, either.