Did you download the driver, and install for the USB connection? USB on a value cutter is not True USB like higher end cutters. After you download the driver, reboot your computer. It creates a com port number in your computer device manager. The com port numbers have to match in your computer device manager as well as your software. Vinyl cutters are not plug and play.
From UScutter.com support
In most cases, all needed PC drivers will be preloaded in VinylMaster. If you need to download and install drivers for the USB connection to work properly go to www.uscuttersupport.com and select the driver that supports your Windows version.
#12 in user manual.
https://www.uscutter.com/static/PDFs/LP3user_manual_FINAL_07_09_2019.pdf