Step 1: Download latest version Opal Single Event plugin and update for your site.
Link download: http://www.wpopal.com/thememods/opalsingleevent.zip
Step 2: Go to Settings -> OpalSingleEvent Setting -> Add slug for field config.
Step 3: Go to Settings -> Permalink -> Disable permalink( click Plain) -> Save and refesh your site.
Step 4: Enable permalink and refesh your site, then check your link