This literal translation is not grammatical correct as the fish should not have "a" before it. It should be 'she is eating fish with salt'.
if you were to translate "she is eating fish with salt" to french, it would be "elle mange du poisson avec du sel" but it has intentionally put "un" to say that is "a fish"
It is not grammatically correct to say that something is 'not grammatical correct'.
Reme why is it aometimes perfect to say avec du "with some" and now wrong today to say with some salt