Good day @remonChuwi,
The copter should've arrived fully functional and ready to go after assembly as-is, was it not the case?
After flashing the OS and installing the autopilot you'll also have to parametrize it (second part of the comment), but that shouldn't be the problem if you are able to arm it and take off.
Yeah. With no hardware modifications and proper assembly that should be the case, is it not?
Could you try swapping motors around?
Please remember they have an order and position, so you could try swapping 1 with 2 or 3 with 4 depending on the inclination you see when flying it to check if it's a problem with the motors.
Also it would be of help if you were able to record the copter's flight.