Well - you've already been told about the 9V battery issue; but you have a larger issue...
First off, where did you get the motors, and what makes you think they are 9-18 VDC motors? I'm not saying they aren't, but without further information, those motors look more like 3-6 VDC motors (standard small hobby motors). Do the motors have any information on them (manufacturer name or symbol, numbers, etc)? Generally, on those kinds of motors, the information will be molded into the back bearing plate, if it exists at all.
Regardless, those are not gearmotors, and you can't just mount some wheels to them and expect them to move your vehicle. They likely have a speed at their rated voltage of something like 3-6000 RPM, if not higher. At this speed will be their maximum torque, and it likely isn't much. Your battery is probably running low because of numerous reasons, not the least of which is that your motors are stalled out (or nearly so).
You need to gear those motors down to drive the wheels. The easiest way to do this (although even if this does generate sufficient torque to move the bot, that thing's gonna run like a bat outta hell - maybe with some PWM you'll be able to keep it under control) is to redesign the chassis and wheel mount so that the wheels are mounted on their own separate axle(s). Then, take the wheels, and sandpaper their contact surface so that theres a bit of roughness; this will give you more traction. If you can, find some wide rubber bands that will fit snuggly against the wheels to act as rubber "tires" which will give you more grip. Oh - and remove that tape on the wheels, that's doing nothing helpful.
Then, attach to your motor shafts a small, short piece of rubber tubing; r/c engine fuel line with a small inside diameter (get it at a hobby shop) will work well here. Another option would be to wrap the shaft with a bit of friction tape. Still another option would be to remove the eraser from the end of a pencil, poke a hole in the center with a small nail, then superglue it to the shaft of the motor.
Once that is in place, mount the motor in such a way using your tape, or some glue (hot glue is great), and/or maybe some rubber bands, such that the motor is placed so that the shaft (with its little rubber "wheel") rubs against the wheel contact surface (either the sandpapered roughness, or the rubber band). This will form a simple friction drive gear reduction.
The gear ratio is easily figured; measure the size of the wheel on the motor, and the size of the larger wheel; divide the larger size by the smaller size, and that will form your ratio - for instance, if the diameter of the small wheel on the motor is 5 mm, and the larger wheel is 35 mm, then the ratio is 1:7 (because 35 / 5 = 7), so if the motor is spinning at 5000 RPM, then wheel would spin at (?) approx 714 RPM (5000 / 7 = 714.29) - note that if these numbers were real, that final RPM is very, very high; you want an RPM of about 100 RPM tops (even 60 RPM would be fast, 100 will be like a darting mouse).
Good luck, and I hope this helps!