Why I get "Cowardly refusing to create an empty archive" with TAR command?

I encountered this warning when trying to execute the command without specifying the zipped file name. For example:

tar -zcvf directoryName

The warning was not issued when I executed this:

tar -zcvf directoryName.tar.gz directoryName/

The command should be just one line:

tar -cvpf /backups/fullbackup.tar --directory=/ --exclude=proc --exclude=sys --exclude=dev/pts --exclude=backups .

It seems you've split it into two lines.