Virtual Optical drive error


Hello! I currently have Oracle VirtualBox and I have a Ubuntu VM (Virtual Machine). The problem is that I downloaded Xubuntu version 18.04 and I was supposed to use that as a Virtual Optical Disk but unfortunately it showed an error message. That error message is:
Could not get the storage format of the medium ‘D:.disk\cd_type’ (VERR_NOT_SUPPORTED).
What should I do now?