I also made a change in your line. I adjusted the amplification of your optical amplifiers to the amount needed for the line. You have used a 50 km SMF with 0.2 dB/km attenuation. So the overall loss will be 50×0.2=10 dB. So change the amplification of the next amplifier to 10 dB. In the same way, you will have 4dB amplification for the amplifier after the DCF.