Why can’t I watch videos on my iPhone?

It’s just a few episodes of Family Guy and South Park that I downloaded from itunes but when I try to watch them it just has a pop up thing that says “Cannot Open”…

Does anyone know whats wrong??

Thank you.
Thanks for the answers, but if I bought them and downloaded straight from itunes surely they’d be the correct format already wouldnt they??

Also I updated my iphone’s software today and before i did that it played videos but now ive updated it wont play any… do you think maybe i should just reset it to factory settings and the try updating again?

thank you