It sounds like you're trying to add an app that isn't in the Roku Channel Store, at least not for your region. If the app isn't supported in your region (the app developer determines that), then it won't add. Note that just because a service may be supported in a region does not mean that the Roku app is. That's a decision the service/app developer makes, not Roku.
Roku Community Streaming Expert
Note: I am not a Roku employee.
If this post solves your problem please help others find this answer and click "Accept as Solution."