sopc builder

Have a look at the messages you get :

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There are no Nios II processors available which match the values specified.

Please check that your PLD is correctly configured, downloading a new SOF

file if necessary.

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it could be that the fpga has no nios configuration during your try to access the flash via command line, in fact the fpga could be empty or has a design without a nios core inside

or ...

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# Programming flash with the project

"$SOPC_KIT_NIOS2/bin/nios2-flash-programmer" --base=0x02800000 --sidp=0x034410d8

--id=761933568 --timestamp=1268280966 "ext_flash.flash"

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as you use the -timestamp option, the fpga has a different image inside than the one you specified

for the first possible cause. load the expected fpga image and the try again to programm the flash, for the second cause, update the fpga image or the timestamp value or remoce the timestamp option.