The most common reasons are the following:
- Wrong baudrate . Your computer may have selected a wrong baudrate such as 9600. The default baudrate for this printer is 115200.
- Missing USB driver . Most of the time, the operating system installs the right USB driver for the printer. But sometimes, the installation fails. The i3 Plus printer uses a cheap USB-to-Serial bridge WCH CH340G. The driver can be downloaded from the web site of the manufacturer (in Chinese). The file to download is CH341SER.EXE for Windows and CH341SER_MAC.ZIP for macOS.
- Bad USB cable . Try to use another, shorter, USB cable.