Waveshare make a ton of products with similar names, so probably best to link the actual board you mean. The closest I can find is https://www.waveshare.com/wiki/RPi_Zero_Relay
If it’s the above, my guess would be the UART interface (pins 8 and 10 / TX/RX for RS485) is the problem. On Linux UBOOT if it notices any input over serial interface it will interrupt the boot process as it thinks you are trying to change the boot parameters. (Raspberry Pi doesn’t have this problem as it doesn’t use UBOOT).
To overcome the above search how to disable uart / serial for uboot. Raxda Zero 3W device on uart prevents boots