Linux support is still experimental but can be enabled.
If you are on Ubuntu 20.04, you have to make sure, that the latest version 3.something is installed from this ppa: https://launchpad.net/~nextcloud-devs/+archive/ubuntu/client. After that you can add the line showExperimentalOptions=true to the [General] section in the nextcloud.cfg file. The file is located in /home/yourusername/.config/Nextloud. After adding this line the option for Virtual Files should apear in the UI, when you add a new account.