This error message is caused when two different Apple ID’s are used when downloading the app and trying to restore your purchase.
What does this mean?
It's possible you downloaded Viki using one Apple ID, and then later on logged in to your current device with another Apple ID and tried to make the purchase.
Another possibility is that you downloaded and purchased the subscription using one Apple ID and now you are logged in to another Apple ID, hence the error message. This issue is not caused by Viki, but by Apple itself.
To fix this, you can follow the suggestions in either of these two options:
Delete the app from your device and log out from the App Store as well.
Locate the Apple ID you’ve used to purchase Viki Pass.
Log in to App Store using that ID, re-download the app.
You can now Restore Purchase on the Viki app.
Instead of using the Restore Purchase option, you can just go ahead with the Subscribe option once you determine which Apple ID holds your purchase. The store remembers that you have already subscribed. As long as you're using the same Apple ID that you used when you originally subscribed, you will not be charged again. You should see benefits after this.
You might need to delete and reinstall the app once you are logged in to the correct Apple ID account.