This is a two step project:
Step 1: Get the storage
Step 2: Allocate it/make it known to the Linux VM
Step 1:
Shut down the Linux VM
Use information here to assign more storage to the Linux VM
https://www.parallels.com/blogs/resize-your-parallels-desktop-vm/
Step 2:
Use gparted to add the additional storage to the Linux VM partition
Download the amd64 gparted from here https://gparted.org/download.php
Change the boot order to boot from CD/DVD
Set the DVD to point to the location of the iso file.