fix file fstab

How to fix /etc/fstab be come readonly filesystem

If a filesystem has been mounted read-only, chmodwill not work since it's a write operation too.

Try remounting it read-write:

sudo mount -o remount,rw '/dev/sdax'

If the device has a write lock on it (like SD memory cards), you need to turn it off. Hardware locks cannot be disabled by software. Note that the write lock on SD memory cards is located from the sight you see the letters near the up left corner and it looks like a very small switch.
Some filesystem drivers may also not support write operations, this is the case with the older NTFS module supported by Linux. For NTFS filesystems, be sure to use the ntfs-3g driver which should be picked automatically nowadays. If not, you can force the driver with something like:
sudo mount -t ntfs-3g -o uid=$(id -u) /dev/sdax /mnt/
(where /dev/sdbx has to be substituted for your block device and /mnt/ for your destination)