You can email support using the picture below for the address.
Or live chat by clicking “chat online” within the green box and not “get help quicker” as this is a bot.
Reply received today from Sky store in response to my email about the error code:
Thank you for contacting the Sky Store team with your query. I am sorry to hear that you are getting an error message when accessing Sky Store movies on your NOWTV Stick. This is a known issue that has been raised and our team are doing their absolute best to get this sorted as soon as possible. Please bear with us in the meantime and your patience will be greatly appreciated.