Translation:Please turn on the light.
Depends on context really. In most contexts indoors it means electric light. However if you were to go outside and talk about a street light it would be referred to as something different: 外灯 (がいとう, outside light), although you would still be understood if you said でんき. If you were in a science class talking about electricity then 電気 (でんき) would be used.
Quite honestly, I was thinking about turning the electricity on in a house, as one does when one moves in or after one has cut off fuses or circuit breakers to do repairs. I am not a particularly DIY person, but I have several times had to ask my daughter to turn the electricity back on after I had fixed something in a lighting fixture. The light itself, however, might very well be in the off position.
Prepositions are entirely abitrary and culturally specific. The definition of things like "on" and "above" is not at all consistent between cultures. English is terrible for this because we have so many hundreds of commonly used prepositional phrases/phrasal verbs. Think about the difference between "shut down" and "shut up". How many different contexts they have and whether any thing is actually moving down or up.
In French, "ouvre/ferme la lumière" is understandable but it sounds wrong to some (most?) people. Académie Française would say it is totally wrong, and that you have to say "allume/éteins la lumière". Still, Académie Française is not a real authority. I actually don't know to which extent "ouvre/ferme la lumière" is used in my country. I don't hear it often myself.
I'm pretty sure that only the kanji are related, but because there aren't spaces in Japanese, I'm pretty sure Duolingo sees "気をつけて" when you are hovering over "電気をつけて". If you were to say "be careful", it would be pronounced differently as the kanji would be isolated and in a different context (it would be "kiwotsukete").