Update: I attempted about 5 times during the day too add the one night to my existing reservation and each time the last confirm gave me technical website error, the room would then be unavailable for about 30-45 minutes, so on attempt 6 when room came available again I went straight to chat to have my reservation modified, they also got technical website error but was able to escalate to IT team to get it resolved and I finally got my extra night added, chat for the win!