Hi mrenn,
You entered that tournament while competing in the Pro Tier, so you'll remain in the Pro Tier (despite your promotion) for the duration of that event and any other events you entered while still Pro. Master clubs cannot be used in Pro Tier events, so that's why you are seeing that message.
Any future events that you enter after turning Master (or for any regular rounds you play) you will compete in the Master Tier and will be able to use Master Tier clubs.