I am running Xubuntu 20.04 as both my host and guest, and I ran into this same issue. The root cause is that GCC is missing in the guest, which is required to build the kernel modules. You can install it using this command:
sudo apt-get install build-essential Once it's installed, you need to at least attempt rebuilding the kernel modules: sudo /sbin/rcvboxadd quicksetup all NOTE: previous versions of VirtualBox used the following command instead: sudo /sbin/vboxconfig If trying to rebuild the modules doesn't work, try reinstalling VirtualBox Guest Additions entirely (run these inside of the mounted Guest Additions ISO directory): cd /media/*/*/ sudo ./VBoxLinuxAdditions.run Here are my software versions for reference: On the host (Xubuntu 20.04): lsb_release -r Release: 20.04 dpkg -l virtualbox virtualbox-guest-additions-iso virtualbox 6.1.16-dfsg-6~ubuntu1.20.04.1 virtualbox-guest-additions-iso 6.1.16-1~ubuntu1.20.04.1 VirtualBox version from Help->About Dialog: 6.1.16_Ubuntu r140961 On the guest (Xubuntu 20.04): lsb_release -r 20.04 VBoxClient --version 6.1.16r140961 gcc --version 9.3.0 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1873922 Title: 20.04 not compatible with virtualbox-guest-x11: Fails changing display To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1873922/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
