Ensure the Allwinner USB drivers are installed on your PC so the device is recognized in "FEL mode."
The Allwinner A50 firmware is like finding a surprisingly sharp knife in a cereal box toy. It’s not elegant, it’s not well-supported by Western forums, but for the price of a fancy lunch, you get a surprisingly stable 64-bit Cortex-A53 experience that punches way above its weight class—provided you speak a little Mandarin and aren't afraid of building your own bootloader.
4PDA is the best global resource for obscure Allwinner firmware. Search for your tablet’s exact model number + "Allwinner A50 firmware."