Before Shabbat, I set a Shabbat clock to turn on my oven on Shabbat morning from 10 AM to 12 PM. The temperature I set on the thermostat is not hot, only warm. On Shabbat morning at 8 AM I put in the oven a chicken (without sauce) that has already been cooked before Shabbat. After 12 PM, when the oven is off, I take the chicken out and eat it for Shabbat lunch. Any problem?

Yes. The problem is that one may not warm food on Shabbat in/on a fire that is not covered in an unusual way that reduces the heat. This is the reason why one places a blech on the stove before Shabbat—it reminds one that it is Shabbat and not to tamper with the setting, and moderately reduces the heat. However, use of the oven without any type of blech is not permitted. Wishing you a wonderful Shabbat and Yom Tov.