I can't control my DJ application with the DDJ-XP2.

The DJ application or operating system might not recognize the DDJ-XP2 as a controller.

For Mac, follow the instructions in the guide [here].

For Windows, follow the steps in Case 5 in the guide [here]
*The guide is for rekordbox dj users. However, you can fix the problem in the same way when using Serato DJ Pro.

You may also be able to solve the problem by doing the following:
・ Disconnect any USB hubs and connect the DDJ-XP2 directly to your PC/Mac.
・ Disconnect all other USB devices.
・ Quit all other applications.
・ Connect to a different USB port.