You need to specify the amount of storage you want to assign to your file server in GB.
Step 1: Select File Manager
From within Commissioning Console, click on the File Manager tab
Step 2: Specify size of your file server in Gigabytes
In the field labelled File Manager Disk Space (GB), enter in how much storage you want to be available on your file server.
Step 3: Save
When complete, click Save.
Now please read the article Configure Custom virtual machines