Well, that's a safety feature we applied for the growing interest of beginners getting into FPV drones to not get injured during the first take-off or break the quad before even getting it to fly.
How to disable Angle-Mode and get Acro-Mode enabled?
All you need to do is download the Betaflight Configurator on your Windows or Mac Computer, make sure all the drivers are correctly installed (Tutorial on the startup page of the Configurator), connect your drone's FlightControl to the computer and click on the "Modes" Tab. There are also several apps for Android and OTG cables available meanwhile, maybe give it a try as well.
A short video on the different flight modes can be found on YouTube:
You can then either completely disable ANGLE (right side click the "x") or pull the range to somewhere you wanna use an AUX switch on your radio. Don't forget to also set up the correct AUX input and give it a test on your bench with your Props OFF!