The Automated Creative Optimization (ACO) function cannot be turned off at the ad group level. This is to ensure that it can continuously explore, optimize, and deliver the best combinations automatically.
If you want to turn off ACO, you will have to manually pause the ad group. Or, when creating the ad group, you can choose to create an ACO ad group and a regular ad group. This way, you can still manage the regular ad group without having to turn ACO on or off.
However please note that a regular ad group and an ACO ad group cannot be changed from one to the other.