Hi,
please try the following:
> Download the OS High Sierra installation app from AppStore.
Go to Parallels Desktop menu bar -> File -> New...
> Click on "Install Windows or another OS from a DVD or image file", then click Continue.
Check to see if the OS X installation image was automatically detected by the Wizard. If so, click Continue. If the OS X installation image was not automatically detected:
> Open Finder and navigate to the Applications folder.
>Locate the "Install OS High Sierra" application.
>In Parallels Wizard click "Locate manually", then click "Image file".
>Drag and drop the "Install OS High Sierra" app into the Parallels Wizard.
The installation image will be detected. Press Continue.
>A dialog box asking to create the bootable image will appear. Click Continue.
Name the resulting image file and its location, and click Save.
Wait until the disk is created. This may take some time.
Once the bootable image is created, click Continue.
If the virtual machine unexpectedly shuts down, restart it to begin the OS High Sierra
installation.
Continue with the OS High Sierra installation.
Once the OS X is installed, install Parallels Tools.
Let us know how it works.