We're sorry to hear about the issue you're experiencing.
If the issue persists after removing then reinstalling Prime Video and after trying the workaround solution in this thread, we recommend contacting the channel's customer support team to report the issue and get help.
Channels on Roku are maintained by the channel developer themselves. In this case, there's likely an issue within that specific channel that needs to be addressed with an update from them.
You can reach Amazon Support here: Amazon Customer Service Support
Thank you for your understanding.
All the best,
Roku Forum Moderator