In order to disable nonlinearities, you need to disable the nonlinearities in your optical fibers’ properties sections where there is a tag “nonlinearities”. And for the optical amplifiers to have the same amount of amplification as the amount of attenuation caused by the optical fibers, you are going to have to multiply each optical fiber’s length by its attenuation (or loss) and then put the value of the corresponding optical amplifier equal to that.