"Please stop playing the guitar at night."
[romaji] Yoru ni gitaa o hiku no wa yamete kudasai.
Why the particle after よる？ We don't use that in あさ, ひる so why after yoru?
You're right that it doesn't require a particle, so if you submitted an error report it will probably be accepted without one. To me the に in this sentence helps emphasize the specific time at which you should not play the guitar.
Why not よるにギターをひくないで下さい。
The original translation seems unncessarily complicated.
Edit after a week: I know why it cannot be like this.
I need a word for "stop"
Would it be ok to say this instead: 夜にギターを弾くないでください。 ???
Is there any major difference between these sentences?
Yoru ni gitaa o hikanaide kudasai.
Please don't play the guitar at night.
There is a difference, that duolingo wants you to use the verb "to stop".
"Stop playing" is not exactly the same as "don't play".
It is close semantics-wise, but not the same.