You have to free space from your server.
You need to remove or move some files from your server to another device or delete something you don’t need. Your files are stored or into /root or into /var/lib/nethserver/
If you use webtop cloud or nextcloud you have to enter into their folders and remove or download and save into your PC some files to free space.
If there are only emails on your server you have to cleanup mail accounts.