Translation:Please don't practice the piano at night.
You could use either, or both here, but the meaning is slightly different. With は you're setting the topic of discussion to night. You might want to try translating "Xは..." as "As for X,..." or "When it comes to X..."
It might be helpful to imagine using は as sort of like starting in a more general context and zooming in on something - it can have a somewhat contrastive effect.
Here that has the effect of making it clearer that you might not mind if they practice at other times.
On the other hand で gives a sense that the noun it marks is somehow being employed in order to perform the action. It makes it more like "Please don't spend the night practicing piano." - though that has a bunch of English nuance which probably doesn't quite apply as well. Maybe "Please don't practice piano using the night." is closer, even if it sounds weird in English.
では has both of these functions and would still have that politely contrastive effect. A stilted translation might be "As for using the night, please don't practice piano." Perhaps somewhat more natural if long-winded: "As for how you spend the night, please don't practice piano."
Shouldn't do not instead of the verbal inflexion don't be used as the first right answer? Verbal speech that's normal. But given that this is being written and moreover ください is used in Japanese which inplies a degree of formality, it should be logical to have do not instead.