Step 1 :The sample space of the experiment is $S=\{9,10,11,12,13,14,15,16,17,18,19,20\}$ and event $E=\{12,13,14,15,16,17,18,19,20\}$.
Step 2 :The complement of an event E, denoted by $E^{C}$, is the set of all outcomes in the sample space that are not in E.
Step 3 :So, we need to find all the outcomes in the sample space S that are not in E.
Step 4 :The outcomes in $E^{C}$ are $\{9, 10, 11\}$
Step 5 :Final Answer: $E^{C}=\boxed{\{9,10,11\}}$