what TV do u have?

Samsung, i believe u can only install apps that are in the tv store, cannot sideload other android apps.

To be sure, go to samsung thread, tell your model, year.... users can comment the facts.

Samsung smart TVs don't simply allow sideloading apps. One is pretty much restricted to apps available within the TV's app store. To sideload apps....you'd have to be a pro at Agile Development programming and take up Samsung TV dev programming..    Much cheaper and easier to get an external box for third-party apps usage.
Some apps you can.. Follow the link AVFAN provided.. However, you may quickly find out how severely limiting that STB box and it's specs are.. My advice? Get your own more powerful and open Android OS powered box.