Yes you can cook a steak in a toaster oven.
Just make sure the steak you're cooking is fully thawed out before cooking it in the toaster oven because the toaster oven is less powerful than a regular oven.
The steak will just need more time to cook in a toaster oven than a regular oven but it can be done.
If you have a regular oven then it would be best to cook the steak in the regular oven or on a grill but a toaster oven will cook the steak as long as you allow enough time and are patient in waiting for a bit longer than normal to cook the steak.
You can preheat the toaster oven if you want too and it's highly recommended that you do preheat your toaster oven before placing the food in the toaster oven to cook.
The reason you should preheat your toaster oven is to allow the food to start cooking faster since the toaster oven would already be hot and not cold.
However you do not have to preheat a toaster oven before cooking food in them if you do not want too.
But while I prepare my food to be put into the toaster oven I go ahead and get the toaster oven preheated.
That way the toaster oven is warmed up to the desired temperature and the food begins cooking immediately without having it starting from cold.