That's strange, if you'd like to help out you can create a bug report on https://github.com/laurens94/netflix-rewind-browser-extension/issues and we can find out what's causing this.

Make sure you have given the plugin the required permissions! about:addons
Otherwise, if you'd like to help out you can create a bug report on https://github.com/laurens94/netflix-rewind-browser-extension/issues