The software is "talking" at a different speed than the hardware is "listening" (common speeds are 115200 or 921600).
If the STB is "hard-bricked" and won't respond to UART, you may need to force it into Mask ROM mode. This involves using a needle or tweezers to momentarily short two specific pins on the eMMC flash chip while powering it on. This forces the processor to ignore the corrupted internal storage and look to the UART/USB port for instructions. Bootrom Error Wait For Get Please Check Stb Uart Receive
Understanding and Fixing the "Bootrom Error Wait For Get Please Check Stb Uart Receive" Message The software is "talking" at a different speed
This allows the software to catch the initial "Handshake" signal as the BootROM initializes. 5. Using the Short-Circuit Method (Mask ROM Mode) Bootrom Error Wait For Get Please Check Stb Uart Receive