If you are sure that you have entered the promo code correctly, then the only reason why it failed to get activated is that the activation limit has been reached already. There are too many people watching the live stream and just a few codes to be activated. Unfortunately, someone appeared to be a bit faster and had already used the code before you tried to activate it.
We understand how frustrating that must be, but don’t worry. There are a lot of other live streams with promo codes and new interesting events to come :)