Problem writing .SOF plus .ELF files with NiosII ECLIPSE FLASH PROGRAMMER using quartus 18.0 : error 8 during flash writing

Hmm, since you say its working with the NIOS2, I assume you are using the JTAG UART port. This port is different from the JTAG programming port. I suggest you look at the board for another JTAG port that is connected to the FPGA. This JTAG port may be the one for programming the FPGA.

Are you using an Intel FPGA Kit or is it a custom board? Can you share the FPGA schematics if possible so that I can check the connection of the JTAG ports?