Oven digital yom tov

I have a digital oven that shuts off after 12 hours can I at say 11 hours extend the time on yom tov, and if yes then why?

You may not alter the oven on Shabbos or Yom Tov

what is the problem? isn’t that groma?

It depends on the system and how it works. So ovens have a mode for shabbos and yom tov which, when pressing the buttons, they do act based on grama, but in general one should not press anything electric on shabbos or yom tov.