You haven't explained how the motor controls work so I don't know if your code will do what you think it will do.
I suggest writing a shorter sketch that has no IF statements and just makes the pair of motors run in one of the 4 combinations - so that you can try out the combinations without the confusion of the IF statements.
When you have that figured out, then move on to the IF statements. If it doesn't work then you will know the problem is with your IF logic.
I think you need an "H-Bridge", you can build your own or you can buy a computer chip called L293 or L298 (some have letters on the end, like D and N). You can also buy for example, I have a L298N motor driver board (L293D is a lower voltage version) for doing exactly what you want, controlling twin motors independently with the Arduino so it can go forwards/backwards and turn.
For variable names, I would suggest something like LmotorF/LmotorB and RmotorF/RmotorB, for left/right motor and Forward/backward for positive/negative wires/pins.
I think you need an "H-Bridge",
The original post implied that the motors do operate so I assume the OP already has a driver of some sort.
Sure, all he has to do is supply power to them and they will go forward or backwards with an Arduino, but he says he can't turn left or right and I simply suspect that is because he can't control them independently of each other, which would imply no H-Bridge. It sounds to me like they either both move forward or they both move backward but not one at a time which would cause turning.
Why would it need an Hbridge to simply use forward/reverse?
How else is it going to reverse ? (relay with the +/- on separate contacts ?)
If it only has a single ended power supply (as opposed to bi-polar) , you would need an H-BRIDGE to reverse. If you had a bipolar power supply you could control which rail (+/-) supplies power to the motor thereby making it possible to reverse. I don't think it
has a bi-polar supply , hence either turning is accomplished by leaving one motor(wheel) stationary and driving the other OR turning is accomplished by reversing one while driving the other. If you know another way , let's hear it.