According to our licensing policy, an upgrade to Parallels Desktop 11 for Mac is supported from the following versions only:
Parallels Desktop 9 for Mac
Parallels Desktop 9 for Mac Upgrade
Parallels Desktop 10 for Mac
Parallels Desktop 10 for Mac Upgrade
Since your previous version key is for Parallels Desktop 7, please visit the Parallels Online Store to purchase a full license for Parallels Desktop 11 for Mac:
After installing Parallels Desktop 11, you do not need to reinstall Windows or create a new virtual machine. You can use your existing virtual machine.