Ubuntu – Failed to install the Extension Pack /home/user/Downloads/Oracle_VM_VirtualBox_Extension_Pack-6.0.4.vbox-extpack

You can try to install the extension pack using the command line:

sudo VBoxManage extpack install <package file>

Some random things to check:

• Virtualbox version and extension pack version must match.
• Files on /usr/lib/virtualbox must be owned by root.
• /usr/lib itself must be owned by root, too as Uri himself find.

If you installed virtualbox from Ubuntu repositories, just install the package named virtualbox-ext-pack.

sudo apt install virtualbox-ext-pack

This way you can be sure that the right version will be installed, and they will also upgrade together if needed. So the name of the game is to use same version for the packages, possibly use the same repository as well, preferably your distribution's repos. After installation you possibly need to

sudo modprobe vboxdrv

otherwise you may still get configuration errors in VirtualBox settings. Besides virtualbox-ext-pack I have virtualbox, virtualbox-dkms, and virtualbox-qt packages installed, a bunch of gvfs packages, and of course dkms.

(You'd want to go to a PPA and fiddle around yourself only if your distribution's offered packages are missing some features and the PPA would offer a later version with those fatures. But then I'd advise to use the PPA for all virtualbox packages and don't mix and match.)