Downloading ELF process failed while using the external memory present in the board

Have you verified that your FPGA configuration is running. Blink an LED or pulse a GPIO pin. This is just to verify that the FPGA configured properly. You didn't mention if the demo project you started from uses external RAM. If you haven't already, try compiling from source a demo project that uses the external ram and verify that it works. Next, run memory test software on that system to ensure that hardware is working.

Starting from a demo project that you compiled, add and remove components one at a time. Each time run the memory test to verify that memory isn't broken. Save a copy of every step so you can go back if necessary. Keep going until you find a change that breaks external memory. That will give you a starting point for debugging.