Wow a KK2 1.5. Man I haven't used one since 2014 or so.
Most often the cause of the instant flip is having the ESCs connected to the flight controller in the wrong pins. If you have the ESC for motor 3 connected to M4 on the KK2 and the ESC for motor 4 connected to M3 on the KK2, or any other such swap, the copter will flip.
Do all the motors start at the same time? If not, the ESCs are not properly calibrated. When your copter is on the ground or on the bench, the motors will do funny things trying to keep the copter level. You can't trust how your motors act on the ground. All you can really look for is if they don't start at the same time.
Did you calibrate level on the KK2?
Does the arrow on the KK2 point at the front of the copter?
Do you have any props on upside down? Writing always faces up on a quad rotor. Sometimes people like to flip over two CCW props and spin them CW... nope...
Do you have a reversed control on your remote? If you have roll reversed, the harder right you push to counter a left roll, the harder left the copter will roll.
I would look for these problems in that order.
When you take off, you want to POP the copter up about a meter. Get the copter up out of the prop wash. I like to put a dinner plate out in a field of nice, soft, fluffy, grass and take off from that whenever I POP a new copter up. It makes for less repair work.
Welcome to the FliteTest forums
@Learner.