/etc/fstab is used by the mount command which is run when your system boots. Lines in this file inform mount about filesystems to be mounted and how they should be mounted. In order for the system to boot properly, you must edit /etc/fstab and ensure that it reflects the partition configuration you used earlier in the install process. If you can't remember the partition configuration that you used earlier, the lsblk command may be of help to you: