What happens if you run the Arduino/DisplayString/DisplayString.ino example sketch that came with the download from https://www.waveshare.com/wiki/2.8inch_TFT_Touch_Shield#Arduino_Demo?
What happens if you run the Arduino/DisplayString/DisplayString.ino example sketch that came with the download from https://www.waveshare.com/wiki/2.8inch_TFT_Touch_Shield#Arduino_Demo?