I prefer the more proprer form: Mi povis nek ridi nek plori Your form is ok, but sounds like a translation
I think I prefer the form you mentioned as well. This was one of the first things I learned in Esperanto - "nek ... nek" = "neither ... nor" - along with "kaj ... kaj" = "both ... and."
In that case, wouldn't the English sentence say "I could neither laugh now cry"?
Both forms are possible, but the second one is standard:
I could not laugh nor cry. Mi ne povis ridi nek plori.
I could neither laugh nor cry. Mi povis nek ridi nek plori.
I was a bit confused as to why there is only one "nek" in this sentence. I thought that they had to come in pairs or more.
According to PMEG, this usage here is indeed correct.
"Nek" can be proceeded with any ne-* word, thus not requiring two "nek"
The English is a bad translation. However the sentence is rendered in Esperanto (I leave that to the experts) "not . . . nor" is simply incorrect English grammar. The (idiomatic, fixed) choices are "neither . . . .nor . . ." (negative, as here); and "either ... or. . . " (positive, for situations of that nature). Native English speakers do muck this up, but like most idiomatic constructions, this is what we have for options. ("not . . . neither . . ." would be equally flawed).
There's no way to report this: "The English sentence is incorrect" is not among the (three only) choices available.