Hi rcl,
I'm pretty certain you're referring to the July 18-Hole Unlimited Tournament. You're seeing that error because you originally entered that tournament as an Amateur. Even though you earned a promotion to Pro during July you still began that tournament as an Amateur player, so you'll remain in that tier for that particular event.
Since you are still considered to be competing in the Amateur tier for that tournament you are not allowed to use your Pro gear in that event. In order to play that tournament again you will need to go back to your Amateur setup.
Any other events you play you will be entered as a Pro and will be allowed to use all of your Pro gear.