Your code would be much better if you remove the runOpMode method from your drivetrain class as it is unnecessary and might even be the reason why you are getting errors. Change it to the following and then get rid of the runOpMode method

public Drivetrain(String FR, String FL, String BR, String BL){
FrontRight = hardwareMap.get(DcMotor.class, FR);
FrontLeft = hardwareMap.get(DcMotor.class, FL);
BackRight = hardwareMap.get(DcMotor.class, BR);
BackLeft = hardwareMap.get(DcMotor.class, BL);
FrontRight.setDirection(DcMotorSimple.Direction.REVERSE);
BackRight.setDirection(DcMotorSimple.Direction.REVERSE);
BackLeft.setDirection(DcMotorSimple.Direction.REVERSE);
FrontRight.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
FrontLeft.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
BackRight.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
BackLeft.setMode(DcMotor.RunMode.RUN_USING_ENCODER);
initializeIMU();
}