Unable to upload firmware to a new motherboard
Your motherboard is not an MKS GEN L v1.0, it's a MKS SGEN L - unfortunately, a very very naming scheme.
Your board is actually a 32-bit board, and must therefore be flashed with Marlin 2.0, built for the 32-bit board.
The firmware is then updated by placing it on the SD card and restarting the board, as explained in the documentation for Marlin here