HunterHunter:
I finally got everything to work. However, my stepper needs more torque, I think, because it doesn't like to spin up. If I use my finger to get the platform spinning, the stepper will engage and keep it spinning.
That strongly suggests that you need to accelerate it up to your desired speed. Either use the AccelStepper library or implement your own acceleration code. This simple acceleration code may give you some ideas.
Here is what I use:
Nema 17 (unknown model - if you know how I could Identify it, that would help with current limiting)
A4988 driver
Measure the coil resistance with your multimeter.
Where did the motor come from?