Any movie your bought through your Prime account will be listed under My Stuff/ purchases when you open up the Prime Video channel on your Roku. This has nothing to do with Roku. Do you have the Prime Video channel installed on your Roku? Which Roku model do you have? Did you purchase this movie using the Amazon app for your phone? Does it show in your recent purchases? Does it play on your phone? Laptop? Tablet? Explain what you mean by "unable to watch it".
