2 days ago when copying a huge folder from the host machine to the guest VM some of the files failed to copy. I then search the google and the suggestion is not enough disk space in the guest VM. A further check with the free space on the harddisk has confirmed the issue also.
Then I shutdown the guest VM, edit hardware setting and wanted to expand the disk capacity but another message pop up saying
'The called function cannot be performed on partial chains. Open the parent virtual disk.'
Another search in google suggested to delete all snapshot and if there is no snapshot just create one then delete it.
So i just do what is said and then the disk capacity has been expanded as required.