I think what Elaran is saying is that it is a nice attempt, but a wee bit too early perhaps in your study of the Sindarin -- and to be fair, it is a tall order to translate Galadriel's famous lament! I am not sure how I would translate it myself.
I recommend that you study Elaran's proposed translation, and the aforementioned study material. Though I would recommend you to always stay critical of neologisms, as they are by nature prone to change. Only attested vocabulary and grammar survive the test of time.
To give you a few hints, I have outlined a few errors:
Vallen i lais dannar ned i gwae
The word order is rather peculiar. The leaves are falling and like gold, so the adverb modifies the verb. Even if you want to say golden leaves, you should not mutate mallen, which is, by the way, what you would call gold, the metal, and not the golden hue. Also, as you might have noticed, the meaning of ned is contested. A better alternative would have been i lais dannar 'lóriel vi hûl; or, if you want to retain word order, glóriel dhannar i lais vi hûl. Elaran translated this be 'laur dannar i-lais nas sûl using na(n) "possessing, provided with, esp. of characteristic feature".
Idhrinn and arnoediad bedh rovail e gelaidh
în is a Sindarin cognate for Quenya yén, and they are, per definition, very long, so and is redundant. únodui would be a better word in this context. Depending on your preference, you can either use sui or be for "like." sui is seen in Lord's Prayer, whereas be is seen in King's Letters, albeit with a slightly different meaning. Consequently, în únodui be/sui rovail gelaidh!
Idhrinn be huith lim ledhenner
Apart from earlier comments, ledh- is an inappropriate word choice in this context.
Glim e laer dín aer a ríanui
Rían is a name, but I applaud your creativity! :)
Hí man i ylf annin adbannatho?
ad- would employ stop mutation, thus aphannatha. Also, you are using the wrong grammar case.
Ah e Mithdôr i dhúath caeda am ‘wing falthad immen
**Mithdôr is problematic; thindor / thinnor would be better words. The original uses ablative, so o "from" would be better, which would interact with thin(n/d)or via stop mutation: dúath is "night-shadow", not "darkness". am is "up", not "on", which is or, but the original uses the allative, so na would be better. falthad "foaming is a gerund, not adjective.
There is more to be said, but I really do not have time to go into further detail at this time.
I applaud you for this attempt! You are not quite there yet, though, but please continue to study, and more importantly, keep on trying!