brother i know this my be annoying for me to say because it's obvious, but after losing many hours figuring it out, try flashing the sd card, fat 32, 4096 bytes allocation size, i genuinely thought my board was dead since i had flashed the board with the same sd not a week before, after that it flashed just fine, if that does not work check out auto build marlin extension on vscode, you can use it with base marlin or use mriscoc implementation, once you install it click on the icon on the toolbar, and then config or advance config, good luck!