Parallels Dektop 12 for Mac is “optimized for macOS Serra”. If you’re using macOS Serra and using Parallels Desktop 11 for Mac, upgrade.

Upgrading to Parallels Desktop for Mac 12

From the top menu “Parallels Desktop”, click “Upgrade to Parallels Desktop 12”.

Upgrade Now Upgrade to Pro Edition

Even if you upgrade to Pro Edition, you can still use “Parallels Desktop 12 for Mac”. See here for other uses and settings of Parallels Desktop.