Put it in download mode. Vol down, home and power button. Be sure to take out battery, sim, memcard out first. Plug battery back in and do download mode. Then install rom again and be sure to select the 512 put file and select repartition options in odin

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that error cannot open port could be your computer also try a different computer or port.

you can definitely flash your firmware with odin. that message just means that odin can not find the device. you have to try it again. maybe restart the device & your pc ...