Error within Bubble Machine coding

That error means you didn't install the AFMotor library:

You should note the documentation says:

This library is old and deprecated - and the hardware disconinued years ago. V2 of the shield uses i2c only and works with anything that has I2C support (e.g. all arduinos) without endless incompatibilities and porting requirements! :slight_smile:
-> Adafruit Motor/Stepper/Servo Shield for Arduino v2 Kit [v2.3] : ID 1438 : $19.95 : Adafruit Industries, Unique & fun DIY electronics and kits