There is no error. Only a warning about "setting GRUB_TIMEOUT to a non-zero
value when GRUB_HIDDEN_TIMEOUT is set". Just don't do that. Edit
/etc/default/grub (here with GEdit):
$ gksu gedit /etc/default/grub
Read
https://www.gnu.org/software/grub/manual/html_node/Simple-configuration.html#Simple-configuration
to understand what variables you can define and how. Those are Shell-like
variables: easy.
Once you are done, save and quit the text editor. Then, execute:
$ sudo update-grub
You can first copy /etc/default/grub somewhere in your home directory if you
are afraid to screw up everything.
