Try a different USB cable or a USB 2.0 port (USB 3.0 ports sometimes fail with older Allwinner drivers).
The Allwinner A50 is a highly integrated, cost-effective applications processor designed for entry-level tablets, AIoT devices, and digital signage. Unlike flagship SoCs, the A50 relies heavily on OEM-specific builds of Android (typically 9.0 Pie or 10 Go Edition). This fragmentation means there is no single "universal" firmware file. Instead, the correct depends entirely on your device’s brand, model number, and PCB revision. allwinner a50 firmware link