I ran into the same issue with my NanoPC-T6 LTS 2310. The JTAG signals don’t appear to be exposed on the standard headers, so connecting a J-LINK directly may not be practical unless you can access the test points or modify the board. Before opening it up, I’d check the schematic and ask on the
Fnaf platform—someone may have already identified the pads or a workable UART/USB debugging alternative.