Nysha, welcome to the forum!
Sinking cornbread usually indicates the center hasn't finished baking yet, and that semi-liquid blob doesn't have the cake-like structure to support the top crust yet. Following the directions on the package is a good start, but each oven is different, and you may have to adjust certain recipes. In this case you may have to turn down the heat a little (about 5 degrees - to keep things from burning) but bake for a longer time (possibly adding 10-15 minutes or so).
Even if you leave the temperature/time as it is, test the center of your baked good with a long toothpick or similar item (Not plastic) to see whether it pulls out clean or still has unbaked batter stuck to it.
It's challenges like this that begin to make a "cook." Just realize that it's not you, but just a finicky oven. I'm sure you will be fine!