If, like me, you use Parallels Desktop Lite, then I have bad news for you. It's no longer free to run macOS or Linux, so DON'T update it if you still want to use it!
In the most recent update 1.4.0, released yesterday 21 Jan 2019, Parallels Desktop Lite has become subscription-only. On running the app you'll be greeted with this message - you have 1 year to migrate or pony up.
The App Store description has been updated to say:
Running virtual machines requires an in-app purchase of an auto-renewable subscription (1 year).
This is probably the doing of Parallels' new owner. Corel Corp., and its parent Vector Capital, aquired Parallels on 20 December 2018.
First PSA - the version on the App Store is still labeled as "Lite" for $79.99 per year, which according to this Parallels KB, has limitations compared to the "standard" version avaialble directly from Parallels, which also goes for the same price.
Second PSA - if you have already updated it, then use Time Machine to revert to the previous version. The last version I have is 1.3.3 (19752). Hopefully you have a copy, otherwise you are out of luck.
Final PSA - An open source alternative is VirtualBox.