This usually happens if you are using a USB 3.0 (blue) port.
Connect the plotter to power and turn it on. Assemble the stand and load the vinyl material if you plan to do a test cut. As one manual advises, "place machine in a stable area that is free of strong vibrations, electromagnetic field, dust, moisture and /or direct sunlight". cutting plotter 721 driver windows 10
Another program (e.g., a Bluetooth service or printer) is holding the COM port. This usually happens if you are using a USB 3
Many cutting programs communicate directly with the hardware using the or GP-GL command languages, bypassing the need for a standard Windows printer driver. cutting plotter 721 driver windows 10