The ESPN app on Roku is not one to which you can subscribe. It is free to download and install but in order to watch regular programming as you would see on the ESPN family of cable/satellite channels you must authenticate the app with "tv provider" credentials, i.e. logging in using your email and password from a cable/satellite subscription that includes ESPN. This is how ESPN gets paid, out of the fees you pay the "tv provider".
Another way to pay for ESPN is by a paid subscription to a streaming service that includes ESPN as part of its package. Examples would be Directv Stream, fubu, Hulu plus Live TV, Sling, vidgo, and YouTube TV.
ESPN does have an "add on" premium service that you CAN pay to subscribe to, called ESPN+. Within the ESPN app, some programs will have an ESPN+ banner shown in the upper left corner of the program's thumbnail image. These can only be viewed if you have an ESPN+ subscription. I don't have an ESPN+ subscription so I only know what I've read. If you have no "tv provider" account for ESPN, but you do have an ESPN+ subscription, my understanding is you only can view the stuff marked as ESPN+ inside the ESPN app, not the "regular" ESPN stuff without the ESPN+ banner.
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