How to set up folder sharing with Lubuntu guest and Ubuntu host?

I've been using VirtualBox, currently at VirtualBox Graphical User Interface Version 6.1.16_Ubuntu r140961, for a while with several iterations of Ubuntu, currently using 20.04 as the host and two guests, one Ubuntu 20.04 and another 21.04. I've set up file sharing with each by selecting "Devices|Shared Folders|Shared Folder Settings" and then creating a Machine Folder using the dialog "Add Share" to set the shared folder on the host and then checking the Auto-mount and Make Permanent boxes. In the Ubuntu VMs I then see the folder in the Places sidebar of Nautilus with its name sf_. However when I tried this in a new VM containing Lubuntu 21.04 I don't get the shared folder in PCManFM-Qt's Places sidebar, nor is there a folder created in /media as happens in the Ubuntu VMs. I have the correct Guest Additions installed.

What other steps are necessary to make folder sharing work in the Lubuntu VM?

Tìm thấy giải pháp. Không giống như Ubuntu, chỉ cài đặt Guest Additions là không đủ. tôi cũng đã phải làm Sudo apt cài đặt hộp ảo-khách-utils